ched208

I just upgraded my Vista 64 to SP2 and now I get the following message on boot up:
Windows Defender
"Application failed to initialize: 0x800106ba. A problem caused this service to stop, etc., etc.,"
restart does not help and I found the service, it is closed but do not know how to start it. I don't see an option to start. I tried to download a fresh copy of defender from Microsoft but I get told I don't need to install. I can't remove defender... it does not appear on my list to add or update software.
What must I do?