Do you have this problem whilst your are completely disconected from the internet?

Sometimes download trojans and things can cause this, if you find that being disconnected from your internet solves this then you may be ok.

The other problem may be your graphics card, if you have a faulty grapghics card you can get this very problem. I have experienced it my self before. You have to methods of solving, download and re-install the driver hoping its the driver and not the card OR buy a new card / get it replaced.

Hope this helps,
Chris