My friend installed Vista over Windows XP. The system gives the option to boot either from Vista or XP on a SATA HDD.
I tried doing the reverse. I have an HP Pavilion with Vista pre-installed. I changed the HDD Controller from BIOS to IDE from SATA. After installing XP, I never go the option to boot from Vista or XP. It boots from XP by default and HP does not provide drivers for XP anymore for the desktop I have. :'(
I have no sound now. How can I get rid of XP ?

Well instead of getting rid of XP totally, you can try to get what you originally wanted, by downloading a trial program called easybcd and using it to put back vista in your bootloader.

I can't exactly tell you how to uninstall xp, but if you want to fix your vista bootloader pronto, pop in the vista dvd and do a start-up repair. After that it will boot back into Vista. Please note however that XP will still be on your system but unusable since you cannot access it.

Thank you for the help. However, I have a question.
The EazyBcd thing and the Vista Startup repair....are those alternatives to each other or do I have to do both - installation of EasyBcd and Startup repair of Windows Vista?
1. I originally wanted to be able to get a choice of booting from either vista or XP. Will that be possible if I install that EasyBcd thing?

Yeah EasyBCD will do that. Using the vista dvd start-up repair will wipe over the XP bootloader and load up vista only.

I was unable to install XP as I didn't have a floppy that had SATA controllers. So, I converted the HDD Controller from BIOS from RAID to IDE. Then I installed XP. Now even if I switch back to RAID, I am unable to use the built in System Recovery nor am I able to a start up repair using the recovery disks that I have. :(

Wait, are you saying it doesn't boot off your vista disk at all? In that case you really should try and get that program. It has worked for me in the past when I overwrote Vista with XP, although you might have to spend a good while figuring out how it works. It's on a thirty-day free trial I think.
Of course, afterwards you have that new problem to solve, not being able to boot off CDs isn't what I'd call an acceptable config. And that's something i can't help with.

Ok a couple of things here, when installing MS os's it is easiest to install oldest to newest. So for example if you want to install xp and vista as you stated installing xp then vista would give you the easiest results. This is due to the way the bootloader works. If you do it the reverse order vista can't use xp's bootloader which gets written to the drive during the install of xp. It is however pretty easy to fix the bootloader problem if you do them in the reverse order. Just do a search on your favorite search engine for "install vista after xp" and you will find pages of info posted by nerds like me. I found this link to be pretty good but your results my turn up something better. With all that said I think the issue you are really having doesn't really have to do with the bootloader. I think it may all have to do with what you stated in a subsequent post

"I was unable to install XP as I didn't have a floppy that had SATA controllers. So, I converted the HDD Controller from BIOS from RAID to IDE. Then I installed XP. Now even if I switch back to RAID, I am unable to use the built in System Recovery nor am I able to a start up repair using the recovery disks that I have."

When you stated that you went from RAID to IDE, you may have changed the way the drives are setup at the BIOS level of the computer. Typically if you have them in a RAID set such as RAID 0 or 5 and on boot partition changing it to something else in the BIOS would render that RAID set useless. The other thing I noticed that you mentioned that's a big red flag is that you didn't have the SATA controller drivers. For Windows xp and Vista for that matter if you don't have a motherboard (SATA Controller) that's in MS's hardware compatibility list there will be no native drivers to even allow you to see the drive during the install. Vista seems to have a few more drivers than xp but the easiest thing for xp is to go to the motherboard makers website and look for the "ide/sata raid driver disk" This will basically be just a couple files that fit on a floppy. In xp if you hit f6 immediately as setup is starting, you will get prompted to add additional drivers about half way through the initial start of setup. Note that you can only use a floppy in xp it is hardcoded in the setup as far as I know. I have done an install of vista to a raid set but the drivers were already there. I think it's possible to use different media(flash drive or cd) to load those drivers. Now for the final thing that I think could be in the mix here. If you happen to have an additional drive in the machine you may be seeing that as you start your installs, which could make it rather confusing as to whether or not you are installing to the drive you originally wanted to. For example in one of my computer's I have on IDE drive and two SATA's in a RAID 0 configuration. I have the exact setup I believe you were looking for, that is xp and vista in a dual boot setup. However when I installed xp I wanted it on the SATA drives so I had to get the floppy and hit f6 during setup, but for Vista it had the driver so I had two drives to choose from (IDE and SATA) If you wanted to install to the IDE that would be fine you just need to select it at the beginning of setup. If however the driver's weren't there in either os you wouldn't see the SATA drives because setup has no driver to be able to see them. Anyway good luck if you haven't already figured this all out. I just had to post because it seemed like some of the other posts had you headed in a weird direction.

Ok my story goes like this:

I attempted to install XP on a pre-existing Vista Computer. But it kept blue-screening just before the installation initiated after the setup went through loading files. I ran a complete diagnostics but could not find any issues with any of the H/W in my computer. I later figured out that the specific error code means that there was "no drive found". I suspected that it was because XP didn't have the drivers for SATA HDD. I changed HDD controllers from BIOS from RAID to IDE. XP installed great. But Vista got lost. EazyBCD couldn't do anything.

I tried to repair using the pre-loaded restore partition - no go. I tried using HP's recovery disks but they couldn't restore Vista's boot partition.

Then I used a vista disk of one of my friends and attempted a repair by booting from it. It repaired and I could boot from Vista now!!!. Cool. But my XP was no more to be booted from. I used the EazyBCD now once booted inside Vista Operating system and set a dual boot. I was able to get the option to boot from Vista or XP.

I then wondered if using the HDD as a RAID would be faster than using it as an IDE drive (I have only one HDD)I downloaded the SATA Controllers from Intel's website. I have a USB drive. But during XP installation, I get the message that Setup could not find a floppy drive in the computer. :'(

Anyway, now I got what I wanted. I have XP and Vista. I don't find any difference in the speed. I think I can live with it.

Anyway, I thank everyone for their inputs as I had to assemble the jigsaw of all your suggestions to get to this point. :)

What is the boot partition.. if you are installing XP and Vista on the same boot partition whichever is newer will work... because Vista has the different booting sequence and use bcdedit however XP has the boot.ini