Hi newcomer and welcome to the Tom's hardware forum.
1- You need check in the manufacturer's web page of your PC if exist drivers for XP.
2- If u can do a dual boot, u need delete the win 7 partition, install XP and install win 7 again.
3- If u only want install XP over win 7 (thing that IMO don't have sense, at least that you have a very old rig)boot from the CD/DVD, delete the current win 7 partition and install XP.
So, What PC do you have?