Replace windows 7 with xp

To do this, you'll need a full copy of XP, and a PC that will have XP drivers available for download.

If you have these, you'll want to boot from the XP disc, and format the windows 7 current install, and install XP.

That being said, is there a reason you want to go back to XP? It's reaching the end of it's life, and will be unsupported in 2014.