Its a cpu problem. game cant run on more than 4 cores.  It a problem if you have a processor with hyperthreading that has essentially 8 cores (4 virtual ones).
To fix this u have to find your hardware_settings_config.xml file.  My save file vas in my documents/my games/dirt rally/hardwaresettings/ Open this file with notepad and replace lines of code from <cpu> to <cpu> with this:
<cpu> <restrict_if name="cores" op="int_eq" value="4"> <restrict_add> <threadStrategy workerMapFile="system/workerMap4Core.xml" forceFeedbackProcessor="3" dvdStorageProcessor="3" dataSetMonitorProcessor="1" renderProcessor="0" updateProcessor="2" fileStreamProcessor="3" /> </restrict_add> </restrict_if> <threadStrategy workerMapFile="system/workerMap4Core.xml" forceFeedbackProcessor="3" dvdStorageProcessor="3" dataSetMonitorProcessor="1" renderProcessor="0" updateProcessor="2" fileStreamProcessor="3" /> </cpu>
Save the file and run the game. This will force the game to run only on 4 cores.  Cheers =)