Question for the computer guys

[blageo23]

Computer one: Windows vista business 40gb hard drive
Computer two: Windows XP Professional 250gb hard drive

Both computers have the sticker on them with the product key and whatnot.

Computer one is all around better minus the OS and the hard drive. Can I just swap the hard drives and have computer one with xp and a 250gb hard drive?

I actually already swapped the drives and computer one boots up with XP. But is that all I really have to do or is there more to it?