I don't know why you'd get any message from Windows Defender when you go to
Windows Update.
However, if Windows Update isn't working right, that's what you need to
troubleshoot.
http://support.microsoft.com/kb/818018/en-us
is the omnibus troubleshooting document--it looks pretty scary, but you can
take it a step at a time--it's very clear.
Windows Defender updates either via Windows Update, or via AutoUpdate.
Automatic updates within the program are done using AutoUpdate. If you are
on a corporate network where AutoUpdate connects to a local server, that
server needs to be running WSUS, and the administrator must have enabled the
definition updates to be published.
--