There seems to be a problem with updating Windows Defender when using Microsoft Update instead of Windows Update in Windows Vista. The problem becomes apparent when Windows Defender puts up a warning in the system tray that its signatures are not up to date.
The problem got my attention yesterday, when KimJ told me he could'nt update his Defender definition file, and I realised that I actually had the same problem - Did a bit of research and found an article which shows how to solve the problem.