yes you can do this. i have two hard drives in my computer, one with vista and one with xp. install vista on the main hard drive, then remove the hard drive and install xp on your "new" hard drive. after that you change the boot settings on your bios. when ever you want to use the slave hard drive os simply press your boot sequence when you load up your computer, i.e on mine its f8.
hope this helps
 Also I never had to change anything in the bios, makes things so much easier lol.
 Also I never had to change anything in the bios, makes things so much easier lol.