Also, you might want to make sure you do a full install rather than run off of disks. If you haven't done that already, it'll just be easier.
You also might check the readme.txt file to see if there are recommendations for changing your graphics settings. I know many graphics cards work better when you turn off the 3D effects in the game.
A new install is still the best advice.
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Most Subtle Official Straightman of the Laughing Hyenas -- Grammarian Brigade.