The OP asked about removing win7 and installing XP, not dual-booting. OP, that is fairly simple, but you will lose all programs currently installed on your PC and have to re-install them. You must back up all of your data first.
You also need a valid XP DVD (or did it fit on CD back then). The way to do what you originally described is to erase your whole system drive and install XP on the blank drive, assuming that you have an optical drive.
It's a pretty drastic thing to do to your new system. Run through the options above first.