Windows System Assessment Tool fails

Source

You can try to delete the most recent files from C:\windows\Performance\WinSAT\DataStore. 

This problem arises in Vista with Aero enabled. You state that the problem appears as soon as you click on a RealPlayer video link on a site in your Opera browser. This problem has nothing to do with Opera, but merely with the media player. Apparently, it uses an overlay to display the video, and this disables the Aero theme.
In my media player, there is an option to change the video output method. If you change that to VMR7 or VMR9, videos play without disabling Aero. RealPlayer may have something similar".