It would help if you could post a screenshot because your idea of grainy might actually look great to someone else or even worse than it appears to you. I'm not familiar with your computer and don't know the rest of your specs, but I do have Windows 7 and play with max graphics and the game has always looked great. So it might not be "how it is", but something else entirely.
I'm not sure how it will effect the graininess, but there are some different mods that can change the resolution, color, and overall look of the game. Just search around!