You need to post details on your computer brand, model, and age...
Usually a driver error or change (but not always), this error occurs when you add or restart a component or a driver. It can result from mixing old hardware with new... or such simple tasks as plugging in a scanner or other device after previously working fine.
It can be as simple and frustrating as a driver that is too old, but can result from a failing hardware component such as a card that plugs into a slot on the motherboard... network... modem... audio, etc... and can be caused by a modem card you are not even using. Or any device which decides to fail from age or use.
If you have upgraded to VISTA or Windows 7, or added some hardware, you may have some trouble tracking this down.
So tell us more, please.