This is what's happening when I try to run Garry's Mod in 1280x720 windowed.
I have an Ati Radeon HD4850 if the problem is coming from the video card...
The funny thing is that I can hear the UI sounds when I move my mouse around, and only that window does that, the rest of the desktop is fine.
It's odd because I tried in another game and it doesn't do this.