I'd start by resetting the photoshop cs4 preferences:
Press and hold the Shift+Ctrl+Alt keys just after starting the launch of photoshop cs4.
Keep holding the keys down until you get a dialog asking if you want to Delete the adobe photoshop settings file.
Press Yes because you do.
In that dialog about graphics card acceleration, you can check Don't Show Again
If resetting the preferences doesn't solve the issue, try turning off Enable Open GL Drawing and restarting photoshop cs4
If that works, then most likely something has gone awry with your graphics card driver.
(you might have to reinstall the graphics card driver)
Double check the Nvidia website to be sure you have the latest graphics card driver updates