If you just want to wipe out windows 7, then you need to boot off of your XP disc, and format the current install. You can't do it from within windows. You can do this by either changing the boot order in the BIOS, or selecting the CD/DVD drive from the boot menu on startup.
Dual boot is an option. But again, I want xp on a computer that has 7 on it now. I have a billing program for my business that is not yet supported under windows 7. Is even possible? I'm having trouble finding drivers to supoort xp on 7 box. thanks