It sounds like your video card is dying or overheating.. That's a guess of
course in the absence of any description of your computer and/or its recent
history.

A few suggestions:

1. Open the computer and run it open after cleaning out all dust bunnies.
Observe all fans (overheating will cause system freezing and/or crashing).
This includes the fan on your video card if you have one. Obviously you
can't do this with a laptop, but you can hear if the fan is running and
feel if the laptop is getting too hot. For a desktop, without touching
anything hold your hand close to the inside of the case and feel how hot
things are getting.

2. Uninstall the video card and swap it out for a known working one. If
things are better, discard the original hardware.

If you need more help, please post back with more details about your system.

http://support.microsoft.com/default.aspx/kb/555375 - How to Ask a Question

Malke