Recently I tried to re-install an OEM XP Home onto its original machine and had the same trouble, plus I could not access MS update. Not sure what the trouble was but I'm glad I had my old unused Vista Basic (which cost me $299 at release) that installed just fine. If a malware scan does not help you may have to reactivate windows using the Genuine advantage tool, or use the phone number to manually activate.
Good Luck
Fungi