there's a number of things you can try.
firstly, Repair Permissions on your main hard drive. (do this from Disk Utility found in the Application folder. select the main drive, and press Repair Permissions)
if that does nothing, Create a new account in the accounts pane in System Preferences, log out of you main account, log in to the new one. now install and run the sims from there.
inform us on how you get on.