I noticed after installing Windows 7 on my machine that it no longer goes to screen saver when the machine sits idle for a while. Originally, I suspected this was due to the KVM that I had hooked up. It must be sending some kind of heartbeat signal which keeps the screen from going idle. Some googling revealed there was a known issue with using a MS wireless mouse that can cause this problem. The fix was to install some optional Windows Update fixes that deal with HID drivers. After installing the fix, it started going to sleep properly on my desktop. Unfortunately, I don't seem to have the same update options available for my Windows 7 Professional Dell laptop, so I'm not sure if it's a global fix.
In the end, I discovered that the real cause was, in fact, the old-school MS wireless mouse. I replaced it with a newer one and the problem magically went away on both my laptop and desktop machines.
