Mouse Disappearing

help me please, all of a sudden today my mouse keeps disappearing. When I am moving my cursor around the screen sometimes the pointer will just not be visible! Especially when i try to press the back button in safari, it will still work but I can't see the pointer, so if I click and its over a button it will click it, but I have no way of telling where it's at! I have the standard white mouse that comes with pmacs if that matters.

please help me?