There are a number of reasons you wouldn't want to try to install XP in dual boot mode on a PC that was built to run Windows 7, not the least of which is lack of drivers, SATA hard drives, booting problems, and others. Better approaches would be to upgrade whatever it is that you think you need XP for, or try to install it in compatibility mode. If neither of those will work, there's XP mode which is free if you have Windows 7 Professional, Enterprise, or Ultimate. If you have Home Premium, you could upgrade to Professional for around $90 and get XP Mode free. Bear in mind that MS is pulling the plug on the last remnants of XP support come April 2014.
Good luck.