Maybe you're short on RAM? The People who create the SIMS often push this to the very limit

When is the last time you updated the sims 3? Make sure you're getting all the updates from the sight, because the original sims 3 came out before late night, and might have some things that need to be fixed before their codes can work together

Check for a virus

Restart your computer. if it still isn't working, uninstall and reinstall both games.
Hope this helped!