strange lines when going to full screen video on 20" LCD

My Dell 2001FP works fine with my 6800GT. except... when I go full screen with a video (media player, winamp, powerDVD, pretty much any video app). I have the latest drivers and divx codecs installed. this only happens when I double-click to go to full screen mode. does this have something to do with my 4:3 aspect ration (1600x1200)?

any ideas?

thanks

sorry i forgot to include a better description...

there are straight lines that run across the screen from left to right and only appear on what is being played,,, not on the status bars at top or bottom.

I found a solution but not sure how to fully remedy the problem. the lines go away when I set "Hardware Acceleration" to half.
it shows: "Disable all DirectDraw and Direct3D accelerations, as well as all cursor and advanced drawing accelerations. Use this setting to correct severe problems with DirectX accelerated applications."

I want to find the root cause of this problem as opposed to doing a bandaid fix. any ideas what is causing this? I have reinstalled directx9c with no luck. I have yet to reinstall my drivers.
thanks