you could try disabling any other software running in the background, e.g take unnecessary stuff out of the startup by using msconfig in run, and unselecting stuff you dont need to be running. Then ensure you disable anti-virus and firewall before starting up the game.
Start at the lowest res in the game, and build it up to an acceptable level in terms of performance vs quality.
I think your pc covers the requirements easily enough, so its strange that it doesnt run too well.
I'm guessing that your graphics driver is up to date.
Hope this helps.