Based on some research I've done on this topic, I learned that with certain video cards, Windows 7's power saving settings reduce the voltage to the video card too much when it's idle.
Many 3D cards require a minimum voltage to be stable, but when "idle", they are basically asleep so everything should be ok.
However, now that so many apps are "hardware or video accelerated" (like Flash, Quicktime, Java, Firefox 4, IE9), the video card is being used when it's in "idle mode" and under-voltage.
The only way to fix this is to tweak advanced settings using 3rd party tools (and possibly fry the video card), or wait for MS to patch this, if ever.