WIN XP stations can't shut down or log off??

Hello all.

I have about 6-8 workstations that will not shut down or log off. This was
an isolated event and I had 1 workstation that was doing it for a while now
it seems to be spreading. Network is virus and adware free and so are the
workstations. Was there some sort of update that would have caused this
problem?? I don't or cannot understand why it would start with 1 system then
over the past 3-4 days I now have 8 that will not shut down or log off using
the windows shut down screen.

Any help would be greatly appreciated!!
Mark