That could be caused by a number of things; let's see if we can narrow down the possibilities:
1. What version of Windows are you using?
2. Does it exhibit the same problem when you boot into Safe Mode? (you get to the safe mode boot option by hitting the F8 key as your computer is starting up)
3. When did the problem start? Give us some history.
4. Did you make any software or hardware adds/removes/changes around the time this started to happen?
5. Did anything potentially harmful happen to the system (power outage, severe crash, etc.) just before the problem began to happen?
6. Get the computer booted up succesfully, get the most current updates for your anti-virus software, and run a full virus scan of the system.
Give us feedback on the above and we'll take it from there.