Does the whole computer freeze? Or is it just the game?
If it's the game I recommend to un-install (make sure you delete ALL the folders, but save presets and maps that you might've downloaded somewhere) it and re-install it again.
If the whole computer freezes then I might think it could be the graphics card. Do you have alot installed on the computer? Is there some things that you can remove from it to make more room on the hard drive (try to defragment your harddrive) Have you run your Anti-Virus program to make sure you don't have alot of malewar and spyware on your computer?
Try some of these things and see if it helps.
Good luck~ // Shallow