Also, is there a way to force the computer's default OS to XP if I don't pick which OS to use at bootup? I only want to run Vista once in a while, not all the time.
Boot your computer in to Windows XP.
Ensure you have the Vista DVD in the DVD drive.
Go to “Start” and “Run”. Type "cmd" & Enter.
Type “e:\boot\bootsect /nt52 c:" - Press Enter
(without quotes, and replacing e: with the drive letter of your Vista
DVD & replacing c: with whatever is your XP partition drive).
***Ofcourse before doing this keep a backup of you data just in case
the boot manager gets hosed. It shouldnt, I have done this many times
and it will restore the boot manager.