The problem with installing XP on a laptop that is shipped with Vista is Drivers. If the laptop was made for Vista (has a Vista sticker on it) there's a 99.99999% chance you won't be able to just slap XP in the drive and install it. You will have to mod some ini's and get some third party drivers for you to even get close to it to working.
The other thing you have to worry about is does your BIOS have the ability to run the HDD in compatibility mode? If not, it's going to be that much harder to get XP on there.
All in all, it's not worth it. If you are getting sick of Vista's little pop-ups and incompatibility issues, then try this, it should solve most of your problems:
Open up CMD (Run -> CMD) type in "net user administrator \active:yes"
(without quotes)
Log off, and then log back on with the option that wasn't there before
It will solve all your problems with access rights and PunkBuster and many other issues that involve admin access. It made a world of difference on my laptop.