Actually, this should be relatively easily solvable if the game hasn't always done this.
First, uninstall the game completely, and reboot your computer.
Then go to the directory where you had the game installed, and move any saved games you want to keep to somewhere else, and then delete the directory. This will insure, with the greatest certainty possible, that no corrupted files will be left over.
Next, now isn't a bad time to use some virus scanning software, so that you can be sure that isn't the cause of any system problems.
Now visit the windows update site (http://windowsupdate.microsoft.com) and make sure you have the latest version of DirectX, and any updates you might want to do. If you install anything you will then want to reboot.
Then you simply install the game again, and see if that helps. This usually solves the majority of problems with games suddenly behaving unexpectedly.
If this doesn't fix the problem I think it pretty much has to be something wrong with your system (as in a driver conflict or screwed installation of windows - which isn't very rare). Other than that all I know of is the generally unpleasant things like reinstallation of windows (which usually goes reasonably well), or the be-all-end-all 'fix' of formatting your drive and starting over, or just installing windows on another partition. But none of that SHOULD be neccessary, and hopefully your problems with be fixed by a simple uninstall and reinstall of the game.
~SDC~