I am using an older computer to play warcraft 3. The menu looks ok, but when I get in game, there are small blocks on the screen. It's like I am seeing through the three d models to the background. The artifacts, a few pixels by a few pixels, are black and brown.
I tried it at the max resolution, and went down a bit to see if it would fix the problem.
This is called artifacting - a possible cause would be overheating. Open up the case, remove the dust in the case AND the GPU. Use q-tips. Don't touch any integrated chips. Better yet, buy a can of compressed air and blow dust out every few months.
If that doesn't do it, disconnect power cord, and reseat everything except cpu fan.