The windows screensaver/hibernation/stand by feature is an NVidia driver issue, as stated before.
It occurs when you change the power management settings from the default settings. If you keep the default settings, it won't happen.
However, if you want to use your own power management settings, like I do, then there is a small fix I discovered. After returning from the screensaver, system standby, etc., your mouse is still functional. Simply right click your desktop (minimize everything or go to start-->Show Desktop if you have to), then go to Nvidia Display --> Laptop Display. And voila, your mouse is fixed. It can be a nuisance at first, but it sure beats restarting the computer or getting new drivers.