Have you tried deactivating the steam overlay in the game's settings within steam? I'm sure I had that issue when I had an ATI card, but I can't remember what I did to fix it!

Windows compatibility mode maybe?

Overlay was already off. And no joy with the compatibility modes.

Could it be a resolution issue? Try playing in a lower resolution and see if it helps. I seem to recall SC having graphics issues when I tried to play it at 1280x1024. Lowering the resolution corected the problem.

Nemo