I had something similar on my 8790. (arrived last monday, loving every second) I didn't see a description of your harware setup, but mine is:
8790 3.0 Ghz Nb
1 gig ram
60G hd 5400
It happened to me once was while playing Quake Team arena, and once was just during initial Windows boot up. The screen is scrambled, and moving the mouse only makes it worse.
Mine was from trying to adjust the brightness of the screen with the function key and the F8 F9 keys while the processor was being used. (i.e. boot up, and 3-d)
The first time, in the 3d game, i thought it may have had something to do with the graphics on battery. But when it happened to me again when it was plugged in and just setting up the desktop, it worried me a little.
I think it may have something to do with the processor having too many things to sort through at a time. The second time it happened, I just waited it out, and it straightened out. Since then, i've become more caution about using the brightness functions. I also updated the Graphics drivers, and have not seen it since.
Hope this helps out. If for nothing other than knowing there's someone else out there who has seen what you see.