Why not Win 7? Win 7 is very fast, cheap, and more powerful than XP in alot of ways. Also, are we talking windows xp pro or home edition, and will it be 32 or 64 bit? If you want to put Home Edition 32 bit on your machine I wouldn't even bother.
Please be more specific, citing system specs, brand & model number (if it is a manufacturer built pc), and the specific problem you are having (error message/code) if you have already tried installing XP unsuccessfully.
I believe you will find that most here will strongly recommend Windows 7 though.