LMAO.
Never heard of WGA popping up mid SP2 installation but I don't think its WGA's fault. Its probably a bad SP2 installation that might have been incomplete or might have been corrupted whilst downloading (But AFAIK, the installer checks for corruptions.).
Wait, when did you restart? After WGA installation or after SP2 installation?
However, it could also be this small bug which had an Update that went like "Windows XP doesn't boot after SP2 is installed". I'll try finding that one and get back here.