Hmm, this is making me think its either a codec problem, though, I'm expecting you wouldn't have this much trouble with them.
Or its something to do with how your video card is playing back media video files.
Go to the Control Panel,
and go to Display.
Go to the Settings tab at the top
and then click on Advanced at the bottom on the window.
You should now in one of the tabs find the name of your graphics card, so click on that to bring up specific driver settings.
Now, these settings will depend on video card to video card, so you'll ahve to do a bit of fiddling to find out where this is.
But what you are looking for, is a setting for video brightness/gamma, etc, that may by chance be set off, so that all videos look iffy.
Make sure all gamma/colours, etc are set to standard 0,0, or if theres a reset button, try hitting that.
Then press Apply, and test a video again.
I hope this helps.
I'm sorry i can't really be any more specific.