I have installed the GOTY edition and even tried installing the unofficial patches, which didn't help.
I've tried installing in a different folder (not Program Files), which didn't help.
I tried installing some other graphic enhancer mods, which didn't help.
I tried the lowest in-game video settings, which didn't help (I ran on highest settings throughout during the first install, which ran beautifully smooth).
I tried fine tuning the graphics in my Nvidia Control Panel, which didn't help. I read a bunch of tweakguides to try and understand what it all does and lowest graphics/highest performance didn't work either.
Any help would be greatly appreciated, even if it's just to tell me how I get back to my original settings where it ran some nicely during my first install. If I can get there I'll stay away from mods and just play the game.
It's not a hardware issue as I have a i3 CPU, 4 GB DDR3 RAM, GeForce 8600 GTS video card (not the newest thing I know, but it worked and is more than par for the requirements).
Thanks!