This happens sometimes when you try to install Pets while leaving in your Sims2 folder in My Documents, even if you've removed your downloads. Take out the folder, uninstall at least Pets, if that's what is installed last, then go into your registry and remove it there, because it didn't really uninstall it
, then try installing again. After it works, replace your Sims 2 folder. If that doesn't work, you're going to have to start over from scratch- uninstall everything, and then don't replace the folder until you're completely finished with installing and patching.