This is the method I used --- so I know it works.
Boot up your computer into Windows XP.
Ensure you have the Vista DVD inserted -- but don't let it state installing.
Inside XP, goto "Start" and "Run" and type cmd then type in
"e:\boot\bootsect.exe /nt52 c: /force" (without quotes, and replacing e:
with the drive letter of your Vista DVD), after it's finished -- remove the
Vista DVD.
Restart computer and it boots right into XP, no Vista boot loader menu.
Now you can remove two files (Boot.BAK & Bootsect.BAK) on your XP drive's
root folder (C

and format the partition/drive where you had Vista
installed.
Use your partition software to merge your partitions together if ness., now
you have returned your computer to its previous state, without Vista and
without the new bootloader.
----------That should have done it---------------
Worse case scenario if the above didn't work and you can't get back into XP
( Not likely ) but if not.
Boot to your XP repair console and copy these two files:
copy e:\i386\ntldr c:\
copy e:\i386\ntdetect.com c:\
Then type:
fixboot
fixmbr
Remove CD and restart --- finished.
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