Phew - and you were never going to give up!

I must say that you have followed a very logical course of action. I would however like to mention that considering that the exact same issue occured on two different drives, I would definitely not root this as a software problem. Or even a problem with the hard drive/controller. A few of your points in your previous post caught my attention

and
would seem to imply that the system fan is probably not spinning properly. My thinking is that when you do either of these steps, the unit heats up and causes the fan sensor to try and spin the fan at max speed and the fan starts spinning. When this happens, the unit cools down enough to POST (I'm going to say POST as I classify this issue as No POST rather than no Boot) and work as normal. When you put the system to sleep/hibernate, the temperature is lowered and the fan would cut out and not start up again until it receives full power. This would seem to imply an issue with the fan itself - it does not spin at all when it should be spinning at low speeds, only spins at high speeds

and
would imply that the unit starts up after it has cooled down sufficiently, although I can't explain why the unit wouldn't start after you've let it lie idle for a few hours.

What I would suggest is that when this issue recurs, try holding a fan near the exhaust vents and then see if the issue occurs. Once the system starts up, continue with the fans close to the exhaust vents and then try putting the system to sleep/hibernate and see if you can resume. If the system resumes consistently with the fans blowing, you can definitely suspect the fan in the unit.

Please post back with your results. I am really intrigued with your issue here.

I didn't mean to imply that I had tried all of the things that I listed. They were various suggestions / solutions that I found as I trolled the net looking for similar problems.

The only thing that really worked for me was holding down the delete key when it booted. Somehow that seemed to bypass the problem and it went to a normal boot. The first time I tried it, it took a few tries before it broke the power on / off cycle. Once I got it running again, I left it running 7/24 for about a week and then I had to restart it because a few things weren't working properly. I was surprised that it restarted normally. It has been running 7/24 for about another week since that reboot and I haven't reached the point of any more experimenting yet.

I followed this thread with a post in the hardware forum because it did seem more hardware related. There was a suggestion that a common problem with the soldering of one of the pins on the NVidia chip leads to a bad response during the POST process and that is why it won't boot properly. If that is the problem, then it may be that it is only a problem when the machine is starting cold. This is the most logical explanation that I have for the sequence of events that I encountered so far. I don't know what the Delete key does to facilitate the boot but it seemed to work for me.

When I have the energy, I need to re-initialize my new hard drive partition and try booting with it and the Windows Disk to see if it will start and let me re-create the XP system. Then I need to see if I can boot normally with XP. Since, Hibernation and sleep seemed to get me into this whole thing, I have turned those options off and I will do that in XP as well if I get that far. I need to use this machine for business purposes so I can't afford to take the chance on having it out of commission again. If it works normally without those power options, I can live with that quite easily.

An Update
I post this as part of my ongoing saga in case it may be of value or interest to others.

Once I got the machine running again (more than a month ago), I didn't want to take the chance and reboot it so I just left it running. Windows update was set to automatically do its thing (I didn't think to disable it initially) so it forced a few restarts along the way and they worked ok. I think that the longest it ran without a restart was around 300 hours up to this morning.

I didn't initiate a restart or shutdown myself until this morning (after Windows Update had already restarted it once). I was still running Vista and I tried the restart (which worked as you might expect), then I shut it down and restarted with the power button. That worked OK too.

I checked on the BIOS situation (the HP BIOS update!) and found that there have been updates for the Intel version but the AMD version hasn't changed since 2007 so I have the latest version already.

Along the way, I purchased a new drive and loaded XP onto it. That worked OK for a while until I let it go into Sleep Mode and then it started having the same problem I had under Vista. This morning, I swapped drives again and booted it under XP and that worked OK as well. The combination of the new faster drive, a pretty clean disk without too much on it (and partitioned) with XP makes it run quite a bit quicker than it was running under Vista.

So the one thing that I haven't tried is to shut it down, let it get cold and then reboot. If there is a solder conenction problem (as one person suggested it might be from the video board to the motherboard), it might not work. At some point, I guess I'll have to give that a try. I have turned off the hibernate and standby/sleep options and I will leave those off since it seemed to be a key factor in triggering the problem on Vista and XP.

The Last Chapter
This is ready to be closed even though I still don't understand exactly why the machine did what it did.

I turned off the machine overnight and it booted up normally this morning.

The Recap
HP Pavillion (dv9201) running an up-to-date copy of Vista Home Premium. The machine is a couple of years old. I had the Motherboard replaced previously under warranty because of a wireless problem.

I put the machine into hibernate mode (which I did on a regular basis since it was new). The next day when I tried to bring it back to life, it went into a loop (in the Post process), trying to start up, shutting itself down and then immediately trying again. I let it retry many times but it never got out of the loop. I tried all of the standard ideas like removing the line power and the battery, holding down the power button for an extra long time etc but none of those helped this time around. I messed around with the HP Recovery Manager for a bit and I managed to get it restarted once but overall, it wasn't much help. The only action that was successful in getting out of the loop was holding down the Delete key when starting it up. It took a few tries but that took it to the HP logo and into the Windows Boot.

Since Hibernate seemed to trigger it, I thought that Vista might play a role so I decided to get a new (larger) drive and load up XP and try that. I had been thinking of doing that anyway so this provided the extra push. I did that and I was able to build the XP system and it ran and booted fine. There were problems in finding the right drivers but I did manage to get over that. I did not think to turn off all of the XP power saving options so when I left the machine on too long, it went into Sleep Mode on its own. I immediately had the same Post loop problem when I tried to get it out of Sleep mode and eventually forced a re-boot.

I went back to the Vista drive, managed to break out of the loop again, booted under Vista and then turned off all of the Power-Saving options. I then ran the machine for more than a month without re-booting it myself. I forgot about Windows Update and when I left it (on) overnight, Windows Update came along, downloaded some changes and forced a re-boot. To my surprise, it re-booted OK. I left Windows Update turned on and it processed a number of batches of changes without any problems.

A couple of days ago, I experimented with reboots under Vista and then swapped the drives and booted under XP. No problem in either case. There was still the possibility that it wouldn't boot when it was cold. I left the new drive in with XP and I turned it off overnight last night. It booted fine this AM.

Conclusions and What I Learned

1. There doesn't seem to be any one software or hardware problem that cause the Post loop. If anything it might be a minor hardware issue that Post detects in combination with trying to come out of Hibernate or Sleep mode.

2. There doesn't seem to be enough information or tools to really pin down what is happening in this sort of situation. There are enough dead machine issues (especially laptops) that there is an unsatisfied need that someone should try to fill.

3. The Vendors of these machines should have enought knowledge and experience that they should be able to help but they are mainly interested in revenue. If I had sent in the machine for service, they probably would have re-initialized my drive and re-loaded Vista. That wasn't an acceptable alternative and in the end I didn't need to do it.

4. New drives are cheap enough, that it is worthwhile getting another one to replace the slower and smaller drive that comes standard with many of the machines. I had an 80 gb drive that was almost full. I got a new 250 gb SATA fast drive for around $70. I bought an enclosure for the old drive that lets me connect it through USB and that didn't cost very much. The machine runs quicker, I can still access all the data on the old drive and I shouldn't have to worry about the new drive failing for quite a while (not for sure but probably). If I have a problem with XP on the new drive, I have the XP install disk to do repairs to XP or I can still boot from the other drive under Vista as a short-term alternative.

5. I'm happy with moving from Vista to XP. I stuck it out for a long time but Vista was slow and annoying. It is a challenge finding the right drivers for XP, especially for Video. I did eventually get everything to work correctly. Then I went a step too far and loaded Directx 9. It appears that you can't use the NVidia native drivers and HP doesn't keep re-issuing updated versions. It look slike I will have to force it back to Directx 8.

6. Don't give up too easily. You can often find a solution if you do enough research and try enough of the suggestions. It's very frustrating when something that you've assumed will always work suddenly doesn't. It can obviously have a serious impact on your life / business. I'm now in a much stronger backup situation than I was before and it didn't cost much. We usually only deal with these things when it's a crisis but the option is always there.

And a Final Comment:
In the end I think that this is the same problem that many other people have with HP Laptops not booting. It is claimed that it is caused by a problem with the NVIDIA chip and there is even a class-action suit being assembled for it.

I am still using my machine because I was able to find a bypass. I documented my experience and the bypass on my site. Click here to see it.