I downloaded a small game from another aw site

and immediately after I closed it down (after installing and setting up) I noticed streaks across the screen every time I moved the mouse or there was any movement on the screen. Also, just as a test, I tried playing a game I'd been playing earlier, but the graphics now appeared hopelessly fragmented. Before I opened any of the files from this d/l game folder, I did run a virus check, but it came out ok. Still, for this problem to occur as soon as I closed the game seems too much of a coincidence.
Does anyone have a clue what might have caused the prob? I've got an older computer but with a new monitor.