Post your full system specs first,also does other games work fine ?
BTW,try running the game in DX9 mode,here is how:
go to C:\Users\Your user name\Documents\My Games\DiRT2\hardwaresettings
And open hardware_settings_config with Wordpad
Change <directx forcedx9="false" /> to <directx forcedx9="true" /> and see if it helps or not