Any one of thousands of problems that occur in Windows; we'll need a bit more information before we can identify what the problem might be.

Specifically:
- Your computer's specs
- Whether you've installed any patches
- Whether you've tried modifying any files yourself
- Whether you're having problems with any other games
- Whether you've changed/updated anything with your computer, including driver updates
That will give us a better picture of what's going on and from there we can see if we work out a solution to the problem.