Just overcame the same issue, and did a bit of everything suggested earlier:
Typed in "
net start windefend" at the command prompt (Start key, Run, typed
cmd, then Enter key, until you get the black background with white writing, much like the old DOS prompt);
The command did NOT execute. I then went to Start, Add/Remove Programs, and removed Windows Defender (tried to repair, but this proved unsuccesful also);
Went to
www.microsoft.com and downloaded latest version of Windows Defender, and everything went smoothly after that.
I had noticed, however, when returning to the Defender home page, that the Beta version of Defender I had been using (along w/ Windows Live OneCare) expired on 31DEC2006.
Thanks to everyone for collectively helping me to resolve this problem.