Try lowering the graphics settings and resolution, and switching to Windowed or Fake Fullscreen display mode, to see if that will make a difference.
You really shouldn't be having a problem with those specs, though.

Try doing a clean boot and then test the game. Click Start, or hit WinKey-R, type in msconfig and hit enter; in the General tab, click Selective Startup, uncheck Load startup items (if required) and leave Load system services and Use original boot configuration options checked. Next, click on the Services tab, check the box to Hide all Microsoft services, then click the Disable All button (maybe make a note of which are currently enabled/disabled), then click OK and reboot the computer.
Run msconfig again to switch back to the normal boot configuration.

Maybe try a clean re-install of the graphics drivers, or revert to an older version. See either of the following, for example:

http://www.tomshardware.com/faq/id-2767677/clean-graphics-driver-install-windows.html

http://www.tweakguides.com/NVFORCE_3.html