1. Go to \Users\[Username]\My Documents\My Games\Skyrim
2. Open the SkyrimPrefs.ini file with notepad
3. Find the line bMouseAcceleration=1 (located under [Controls]) (If there is no line, add it)
4. Replace the 1 with a 0

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Turn of AA and AF

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1. Go in start menu.
2. Type in Run.
3. When it will popup type in Msconfig.
4. Then in the Msconfig go in Services.
5. And the find Windows Event Log.
6. And uncheck Windows Event Log.
7. Restart your Computer and your Done.

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Go into your Nvidia Control Panel - 3d Settings, and make a change to Maximum pre-rendered frames to 1 (0 is too little, it affects FPS, 2 and 3 seem to make the mouse much more sluggish, 3 is default).

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Disable any background utilities even your Antivirus. Play offline mode...

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As for you PC, well...Your processor is not bad, but quite week for skyrim (if you want to play Ultra settings + high end mods). Your GPU is good, but i don't thing it can handle well texture mods at ultra settings also.

What you can do. If you don't have much money you could change your GPU with a better one, plus a better PSU. (if you have an old PSU). However your CPU will bottleneck you system a bit. So that's not the finest solution.

I would go for a complete PC overhaul here. Your Mobo is not bad either, but with today standards, a 775 socket is obsolete now - for the reason, it's not upgradeable any more in a matter of performance boost.