No, deleting the cache files did not work. But this did:
Go into Documents or My Documents.
Go into Electronic Arts.
Go into The Sims 3.
Pull out Save folder, Downloads folder and put them on desktop.
Rename The Sims 3 as The Sims 3 old.
Start your game.
It will be like a new game, so quit. Open My Documents again. Open Saves folder and the Saves folder on your desktop. Move your games from the desktop into the new folder. I haven't tried this with the downloads yet, but other people are saying it works.
If you want the Pets demo, people are recommending that you manually install to a folder separate from your games.
Also are recommending you treat the demo as a patch and remove mods and CC (not that anyone on this forum has any mods, just so you know.).