Welcome to the forums!!
Some good advice above!
Also, I would use Driver Sweeper ( free utility ) to clean out any remnants of old nvidia drivers ( or ATi if you previously had an ATi card in your system before you bought the GTS 450 card ) as a precaution here.
Get it here:
http://driver-sweeper.en.softonic.com/Run that and reboot your system.
Download and install the latest correct video driver for your card from nVidia's website:
http://www.nvidia.com/Download/index.aspx?lang=en-usSee if that helps at all...good luck! If latest driver does not work, roll back ( download and install ) to this earlier driver: NVIDIA 266.58 Driver which is known to be rock solid with Black Ops game.
Also, Sauron wants to know if you have a dual core or quad core cpu, it might be a factor here in Black Ops:
http://answers.yahoo.com/question/index ... 049AAcaAahnVidia recommended game settings for Black Ops for GTS 450 card:
http://www.geforce.com/Optimize/optimal ... TS-450-OPSBlack Ops is pretty intensive on high settings, so make sure you close up all other unnecessary programs before launching the game, reduce the load on the cpu as much as you can...am also assuming your system is free of viruses etc...