I have a persistent problem on one of my home network machines in that Windows Update REFUSES to accept that I am an administrator and so have authority to download/ initiate updates. I have been through all of the Windows Update help routines with no success.
That posed a problem in the case of critical updates (as recently). Someone (Xaccers I think) told me to simply go to
www.microsoft.com, select downloads and then to do the update downloads manually. IT WORKS! Try it.