WSUS update problem on Vista SP1 running as a guest OS

I came across an article just last week relating to a WSUS update problem
when running Windows Vista as a Virtual PC guest OS. Just today, I
encountered this problem again, and now I cannot locate that article.
Searches are not producing the correct results either. I am certain that the
fix was the creation of a registry key inside the guest OS, but I cannot
remember for the lilfe of me what that key is.

Has anyone else encountered this issue? Does anyone else know the fix?

Vista SP1 running as a guest on Virtual PC 2007 on a Vista SP1 host.
Attempting to connect to WSUS 3.0 SP1 running on WS03 R2 SP2.

Richard