I would give what telly said a try. If that doesn't work, follow below:
I don't know what kind of video card you have Bossy, but most decent 3D graphics cards have their own programs to adjust settings for graphics. Such as ATI's Catalyst Control Center (CCC) or nVidia's Control Panel.
If you go to START > SETTINGS > DISPLAY, go into the ADVANCED tab, and then select the tab for your video card, you should get access to these advanced settings.
There will be a section for VIDEO/TV and such. In there you can set brightness, contrast, gamma, etc. settings for how your graphics card handles video, independent of everything else.
I posted a picture from my work PC that has an nVidia graphics card so you can see what it looks like:
http://www.fseven.net/images/Misc/nvidiaCP.jpgHope this helps!