If its seperate Hard Drives...I dual boot 2003 server and Vista. What I learned is that when you install XP on the other hard drive, make sure the vista hard drive is NOT hooked up. After your done installing Xp, Then re-hook up the Vista Hard Drive.
If its on one Hard Drive, you'll need something like boot manager or do a google search for something like dual boot master boot record or something .
I always goto
www.computing.net when I have a pc problem..very good people pver there.