Do you have any peripherals attached to the computer while it's trying to go to sleep or screensaver mode, such as a webcam or joystick? Try removing those; there's been a rash of similar issues related to drivers causing this kind of issue. Though not mobo drivers as Cloaked guessed, the drivers are turning up in a variety of mediums, from USB webcams, joysticks, controllers, etc. Look for updated drivers for these peripherals to hopefully solve the issue. Until then, the only fix is to remove the plug from the computer when you want it to sleep or go to screensaver.