Here is a quote from a website about capturing screenshots of videos playing:
----
#I tried to capture Media Player or Real Player and the video is a blank solid color.
When Camtasia or SnagIt grabs a solid block of color instead of an image which looks fine on the screen, the problem is usually due to graphics hardware acceleration being used to display the image. The newer media players from Microsoft and RealNetworks are good about trying to use any graphics hardware acceleration that is available on the system. For example, some systems have graphics hardware which can perform a "hardware overlay" of video on the screen which bypasses the normal Windows display memory that SnagIt captures. Most video capture programs used to capture video from a video camera use "hardware overlay" by default for their video preview. Note that this kind of problem affects all screen capture programs, and even the image put in the clipboard by the PrintScreen key.
The best solution is to disable hardware acceleration in the application that is playing the video, or disable hardware acceleration system wide.
To disable acceleration in:
*Media Player: In Media Player 7, select Tools>Options>Performance. In Media Player v6.4 and earlier, select View>Options>Playback. In both cases slide the Hardware Acceleration slider to None.
*RealPlayer G2: Select Options/Preferences from the RealPlayer menu. Go to the "Performance" tab and disable (uncheck) the "Use optimized video display" setting. You may need to stop and restart any movie being played to have the change take effect.
* For the Apple QuickTime, select Edit>Preferences>Streaming Transport. In the dropdown box select Video Settings and uncheck all of the DirectDraw options.
To disable graphics hardware acceleration globally for all applications on Windows 2000, select Control Panel>Display>Properties>Settings>Advanced>Troubleshooting, on other versions of Windows select Control Panel>System>Performance tab>Graphics>Advanced settings. In both cases slide the Hardware Acceleration slider to None.
----
Personal note, Just make sure you slide it back to full after you finish your captures because in software rendering mode you loose a lot of quality and performance