why not re-install his with the version of windows it came with? Most PCs come with either a windows install disk OR a repair disk that returns it to factory defaults. His machine may even have a hidden partition that you can use to restore from (if you haven't trashed that as well).
RE the codes - of course it matters. Technically, there is nothing stopping you installing your windows on his box, but legally, YOU bought a license for XP Home for your computer and HE bought a (probably OEM) license for XP Media Center.
edit: and here's a link that discusses how to restore:
http://h10025.www1.hp.com/ewfrf/wc/d...cname=bph07143