Had this on my old 8800GTS, started "slowly" as seen on your desktop, but then it just made bigger and bigger artifacts and it was unusable.
In my case it was dead VRAM on my GPU (which basically means blown GPU) and the only solution was to change it.
When googling you will find one workaround - put your GPU to microwave owen for a limited time, it's not a joke and it works sometimes, but didn't work for my 8800GTS, had to change it.
Generally if you start to see something very odd, like this, you should just change the part as there is a high possibility it will come back again later or it just "blows" your GPU anyway... Time to upgrade I'd say!