If you have a NVidia videocard change the 3D settings to "Let the 3D application decide".
Thank you for the answer, I work with a Nvidia FX-580 and the settings are already set to "Let the application decide".
You provide very little information, so...
A link with many ideas about computer setup http://forums.adobe.com/thread/436215?tstart=0
Work through all of the steps (ideas) listed at http://ppro.wikia.com/wiki/Troubleshooting
If your problem isn't fixed after you follow all of the steps, report back with ALL OF THE DETAILS asked for in the FINALLY section, the questions at the end of the troubleshooting link... most especially the codec used... see Question 1
Also... nVidia has MANY card driver versions, not all of which are 100% good with CS5... at least for Win7 64bit
Since you don't provide your OS, you might do some checking to see if your nVidia driver either needs to be updated, or maybe rolled back to a previous version... since there was a recent thread about the latest nVidia driver causing problems for one person, but rolling back to version 197.45 fixed his problem