Vista Won't Shutdown or Restart

Seen this problem a couple of times usually after I've left the PC alone for an hour or less I think--I doze off. (I don't use sleep or hibernate.) Opening files work (haven't tested anything else) but when I select Restart or Shutdown nothing happens. I read that Vista could be in a loop of some sort. Any idea why I'm incurring this issue on occasion?

Thanks as always.

When this happens try opening up task manager and killing windows explorer.

Thanks. If it happens again I'll try that.