Why does dirt 2 does not run smoothly in windows7

What C2D? What video card are you using? What Resolution? What Settings? Neeed more info to give better diagnostic.

If you have a really old video card, then Dirt 2 will not run well. I had the same issue with my Radeon HD 3870. When I upgraded to a 5750, the problem went away.
As Rattman asked, what video card do you have?

i am using nvidia 8400gs 512mb with resolution 1360*768

8400gs is ridiculous card for gaming
my friiend couldn't play Shift on i.So forget dirt2 or upgrade to some real gaming card

This GeForce™ 8400 GS is very low end card contains only 256MB memory and also DDR2 memory type.

This is not a bad card but don't expect to play a very high end games like Dirt2, Metro2033, BFBC2 and etc.... on a very low end card.

8400GS
Stock Clocks : 450
Overclocked Core Clock : 551
Stock Memory Clocks : 800
Overclocked Memory Clock : 940
Highlighted Features : HDCP Ready , HDTV ready , Low Profile Compatible

I think you may play the DIRT2 game on this card at low settings if you over clock it . there is a chance but not sure