I also have a q6600 with 4 gig ram and a 8800GTX 768 MB card. My vista64 is completely optimised (stripped from all the eye candy).
Game runs crap especially in cities. Just makes you cry. :mad:
I had the same problem as the initial poster.
My solution:
I've put my DVD in my DVD burner instead of in my DVD rom. (basically changed CD drive) and the problem was solved.
Seems that securom 'saw' my DVD drive as being an image or whatsoever :-)