If you are using Windows 7, make sure you are right-clicking on the rfactor.exe and selecting "Run As Administrator..." as this has been known to resolve the symptoms you describe.
Failing that, make sure you've done a complete uninstall of rfactor prior to reinstalling, and don't install any mods/addons/plugins until you are sure rfactor is working correctly.
To make a clean install of rfactor:
1. Uninstall rfactor either from your Add & Remove programs option in your Control Panel or using the game’s uninstall feature.
2.
DELETE THE ENTIRE RFACTOR FOLDER along with all its contents and any
rFactor folder located in your Documents folder (Vista and Windows 7 users)
3. If you do NOT have the SafeDisc DVD version which requires the DVD to be inserted to play, then download the 1255 full installer from the download’s page of rfactor.net
4. Install rfactor (with adminstrator rights)
5. When prompted to update Directx, select YES (even if you have the latest Directx, Vista or 7)
6. Launch rfactor. If prompted to activate rfactor, use your 32 digit activation code received from Trymedia and follow the instructions in item #2 at the
trymedia customer support center.