I had this problem too at first until I completely uninstalled, used CCleaner and deleted a few things in the registry. Then I reinstalled it and it was happy.
Do you know what needs removing from the registry? I want to install the actual game (I did have an arr'd version but got the game free so...may as well have the official version) and it keeps telling me it's a unofficial version and blahblah. I've found it in the registry and deleted the 'The Sims 3' folder. Is that all that needs removing? I've run the CCleaner to clean up my registry as well. I want to make sure that is all the needs doing before re-installing to find I need to go through it all again.