If that doesn't work, then you might have some other MS AV like Forefront
running and it the defender install can't tell the difference.
-Eddie
Are you running Vista and is a McAfee application installed?...
Windows Defender error when restarting computers with VirusScan 2009
installed:
http://service.mcafee.com/faqdocument.aspx?id=TS100610&lang=en_US
--
~Robear Dyer (PA Bear)
MS MVP-IE, Mail, Security, Windows Client - since 2002
McAfee is not compatible with Windows Defender.
There is a very long conversation about it here:
-=-