Have you downloaded the Windows 7 drivers for all of your devices? Sometimes sketchy drivers will prevent the OS from entering
screensaver mode or there might be a "minor conflict" that prevents powering down of device.
Set your screen saver to go on in "1" minute so you can quickly test this. Wait 1 minute and see what happens. If Screensaver doesn't enable, make sure that it works by
Right click desktop -> Personalize -> ScreenSaver -> Preview
It should work. If not, reply here with error msg. if any.
So, get the updates from Windows update.... if problem persists, see if device vendors mention this as a problem in their documentation. (IE Mouse vendor website, etc) Be sure to update any BETA drivers!
If this doesn't work or there are no updates, then try Device Manager -> Mice and other pointing devices -> Properties -> Power management ->uncheck Allow the computer to turn off device to save power.
Try for other devices too like network adapters, etc.