I have been documenting my laptop problem in the Vista forum (ref) but it seems that maybe the problem belongs here. The system is almost dead even though there is a way I can break out of it.
In a nutshell, there is something wrong at the POST stage that causes the machine to cycle on and off. I seems to do one quick read of the drive and then just sits there with the lights on and the screen black. After 20-30 sec it powers itself down and then immediately powers up again. It won't boot off a CD when it is going through its power on - power off cycle. Even though it starts to read the CD, the system powers down before it gets too far.
After enough tries, I can interrupt the loop with the Del key at which point it goes to the HP screen and then boots into windows. Once I'm able to get it started this way, it runs fine (I'm on it now).
Without going into the whole saga, I was able to create an XP system on a new hard disk (on this same laptop) and initiallty it was ok but it eventually had the same startup problem as the Vista system.
I already recreated the boot record just in case it was responsible for the problem. At an earlier point I also swapped out the memory sticks and used another pair (which didn't change anything). The fan is working. I haven't reset the CMOS (yet). I couldn't get the plug out and I was afraid that I would damage it or the mobo if I pulled on it too hard.
Any suggestions as to I can determine what is causing the startup problem?