This problem has now gone away.
"Charlie Lewis" <charlie_c_lewis@newsgroup> wrote in message
> One of the updates that persistently fails in this installation of SBS2008
> are the definition updates for Windows Defender. I thought it might be
> that Defender was not installed. So I downloaded the 64-bit version of
> Defender and tried to install it. The system told me this was not
> necessary as the software was already installed. However, I can't find
> Defender on the system, searching for it as you would normally in Vista.
> Any ideas?