Three things to note:
1.) You'll have to purchase a copy of XP yourself.
2.) Be sure to burn the manufacturer's system recovery DVDs before installing XP so that you can reinstall Vista down the road.
3.) Be sure to install XP before setting up the computer as any files and settings will be lost in the installation process.
You can click here for a step-by-step guide.
Hope this helps,
I'm in the market for a new desktop but don't like that I'm forced to have Vista as my OS in it. Seeing as how the selsction of (new) machines with XP is basically nonexistent, can I change the OS to XP with minimal trouble? After reading and hearing all the horror stories, I'd prefer to wait until Vista has all the bugs worked out before going that route.