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