For starters, kindly post a dxdiag so we can take a look at your hardware/driver setup. Use the link in my sig below if you're not sure how to do a dxdiag.
Other than that, there are a number of possibilities. One that comes immediately to mind is that you may have some sort of file corruption either in the game software itself, file corruption in the data folder (stored elsewhere on your computer), or even an addon which may be causing grief. But let's see the dxdiag first, and work from there.
