Just in case it happens to me (or you) again.

1. Open command prompt (or use a for a better console).
2. net stop wuauserv
3. regsvr32 %windir%\system32\wups2.dll (regsvr32 %windir%\syswow64\wups2.dll for win64)
4. net start wuauserv
5. exit


http://forums.cnet.com/5208-6142_102-0.html?forumID=5&threadID=276886&messageID=2660716
http://support.microsoft.com/kb/943144