http://apcmag.com/5485/dualbooting_vista_and_xp

In order to install XP Pro on the machine, you’d need to have a copy of XP Pro, which most likely means buying a copy…in which case you’ll get the activation code for XP at the same time. You can TRY using an old copy of XP from your old computer, but if it’s a copy of XP that was specifically bundled with that machine, you’re probably going to be out of luck, as those XP CDs are often tied to the specific hardware they came with (and if your old machine came with a recovery disc instead of a full-on Windows XP CD, you’re definitely out of luck). Two warnings: if you’re using an old copy of XP you’ll have to make sure that it isn’t also running on another machine, or Windows Genuine Advantage may stop you from using it to the fullest, which you want to do for proper protection against security threats, spyware, viruses, etc. Also, if you use the Vista machine for a while, the only way you can put XP onto the machine is to wipe the drive entirely, so make sure you back up all your data before “downgrading” to XP.