If you can get an XP install disc,
and you have the COA key, you could use that to reinstall. You need an XP install disc of the same version (e.g. Home Premium, Pro, Ultimate) as what you had installed. If you want/need to buy a disc, Amazon has them starting at $75. See this link
I don't think MS is selling XP any more because they seem to think Windows 8 is the answer for everything. To say I strongly disagree with that philosophy would be an extreme understatement! Good luck.