Windows Vista Vista Screen goes black, then Switch User Screen pops up

I have this annoying problem with Windows Vista Home Premium. Every few minutes my screen goes black. If I move my mouse or press a key I get the Switch User Screen. Then I press my user icon and return to where I was working. In a few minutes the screen goes black again. I added another user, and when I'm logged on to that user, it seems to work OK. This problem started a few weeks ago.

Update - I add a new user, and the problem goes away. When I switch back to my original user, the problen persists.