OK, I re-installed and bypassed that problem...only to come to another one.
I tried following EA's rules for manually re-installing my game, but when it came to deleting the registry keys, I couldn't find the keys for some of the expansion packs, so I simply ignored it and moved on.
I tried re-installing one last time, and it was fine..except the Sims 3 Pets icon was just a classic Windows icon, like it hadn't gotten to download.
So I opened the launcher, and got an error message saying that my base game is not compatible with my expansion pack, or something like that?
Wow, I am SO disgruntled over this. I just hope I get play my game again!