What leads you to think that? My Media Center Rollup 2 has been running
Defender since March of this year, including through the April 2006 Update
Rollup KB914548.
Whatever you do, (installing and updateting or unistalling) always as a
adinistratºr.
This is the key!
Can you gº to Windows Update, select Express (Get high-priority updates),
and apply all offered updªtes http://update.microsoft.com
Once that is finished, can you verify the update installs? There are many
pre-requisite files which are needed from windows update and signature
update
with windows defender will fail until you hªve these files.