Firstly, make sure the drivers for the graphics card are up to date. Latest driver should be 8194
Close down anything running in the background.
Easiest way to do this is start - > run -> MSCONFIG
Click the startup tab, click disable all button and click ok. Reboot when prompted. (You can undo this by following the above, and clicking enable all)
Try again, see if the problem re-occurs
if it does, try setting the battle size slider to minimum in the options and see if the problem occurs.