Although this probably won't fix your problem it's worth a try.
Lots of people have had problems with the DX9 drivers in the game.
To show how bad it is, most people have seen almost a 100% gain in FPS by switching to DX7.
You make the change under
Options>>>>Graphics>>>Rendering method preference.
I would also check for normal hardware updates.
Video
Sound
Motherboard drivers
If this doesn't solve the problem, I would move on to checking your system for spy/spam ware.
Typically you shouldn't noticed the kind of problem your describing but it's always possible.
Without a doubt the best way to check your system is with a program called hyjackthis
You can get it here
http://216.180.233.162/~merijn/files/HijackThis.exeUsing "Hijack This" comes with a warning and it's a big one.
Don't use the option to remove anything unless you know what your doing.
It will list everything, good and bad. If you don't know what you're doing it's very easy to screw up your system.
If your not ABSOLUTLY sure about what needs to be fixed, use the option to save a log, then post it in one of the support forums listed here
http://www.spywareinfo.com/~merijn/forums.htmlOr you can email me a copy and I will help you out.