Please post a few details about your system. CPU Gcard and Ram, would be a good start .
.
The minimum spec you see on the box is based on W2000/XP not Vista.
Vista is a hungry beast and although 512m is the minimum Ram requirement. I find 2G adequate but since I got a new monitor 1680x1050 even that is not quite enough with all the settings max'ed out and super sized cities with 2000 or more people.
With Vista it likes to just sit there using 600-800m in the back ground (yes it will run on less) but the point is if you have less than 1024m (=1G) you get vista using "virtual memory" this is space on your hard drive as ram. that's a 100/1000 times slower than real ram.
.
If you mean that the game runs at normal speed but with lots of skipped frames that is a different problem.
.
Without any specs its a guessing game.
