I had issues with this before in Vista, turned out to be some network activity. You gotta change some setting to let Windows sleep/screensaver/whatever even during intense network traffic. Can't remember where it is but I'm sure someone on this forum knows what I'm talking about. Or you could just unplug the network cable/disable wireless when your AFK I guess as a temporary solution/troubleshooter.
I have a MS Wireless keyboard and mouse. The new MS link from stagnant16 fixed he problem of my computer not going into the screensaver mode. Also, my 2 other other computers in the house would not go into the screensaver mode at the same time??? Was it a coincident or did my W7 installation keep the network up??? Anyway, great solution
For what its worth: I have found out that a gadget was my problem in will the screensaver start or not. Whenever I had my radio gadget on a stream the screensaver refused to start. When I turned it off (no removal but just no working stream), that screensaver started without a problem in the given time.
This didn't happen in XP nor in Vista with the same gadget. So it must be a thing with Win7.