I'd go into your antivirus and make sure it isn't sand boxing any of your games. Have you tried running your game with admin permission?
Yes, I have tried running as adminstrator, but still no use. I tried installing another game Burnout Paradise Ultimate and it runs perfectly. But, breaking my head why Dirt 3 doesn't in 64bit. I am suprised that it ran perfectly in 32bit Win7 Ultimate without these admin permissions etc. With regard to antivirus, I have Kaspersky Internet Security 2011, should I make Dirt3.exe as trusted under "Threats & Exclusion" option? Will try that option also today evening when I reach home.
Any more thoughts to try pls....