Do you have a legal copy of Windows XP that you can use to re-install XP? You will need that. You then boot from the Windows XP install disk and perform a full installation (a.k.a. a clean or custom install).
When you do this, everything on your hard drive now (with Win 7) will be gone, including any files you have saved. You need to save anything you want BEFORE you install XP. You can write those files to a DVD or CD, a USB drive, an external hard drive, or to another computer on your network.