Is start by making sure you don't have any specialty hardware that the XP setup might need special drivers for. SATA controllers are a good example. You might also want to try to track down XP drivers for your other hardware before you get too far ahead of yourself. If XP drivers don't exist for a lot of your hardware, you may just be stuck with Vista.
The other thing I would do, is make a boot floppy or CD that contains the tools necessary to partition and format the drive. Vista and XP use slightly different versions of NTFS, so getting rid of the Vista partition and just making an unformatted NTFS partition might be a good idea. Let the XP setup program format it with XP's version of NTFS.
Ok. I am having serious troubles uninstalling Vista, and putting XP over it. I get to the point of the XP screen where it says "Windows is starting" right after it loads up drivers and the such, but before the setup and actual installation.
Every time it gets to this point, a blue screen pops up, known as the BSoD. Telling me that hardware may not be compliant, or that I might have a virus. Any suggestions??