I have a similar problem in XP, yet it occurs with the built-in screensavers. I'll leave and the screensaver will be on, but when I get back, the screensaver has shut off automatically, and there's my desktop which has been static for who knows how long. I originally thought it might have something to do with the sensitivity of the optical mouse.

I'm beginning to think it has something to do with the power saving settings I have for the monitor. It seems like when the monitor is supposed to power down, say after an hour of no use, that's when the screen flickers and the desktop reappears. I haven't looked that deeply into it.

I've resorted now to turning the monitor off whenever I leave.