Try this: copy the code below into Notepad, replace "(username)" with the username you use to log on, and replace Pets and Sims2EP4 with your most recent EP/SP and .exe file. (Note that this assumes you are on Windows 2000, XP or Vista, with a default game installation)
DEL "C:\Documents and Settings\(username)\My Documents\EA GAMES\The Sims 2\groups.cache"
START "C:\Program Files\EA GAMES\The Sims 2 Pets\TSBin\Sims2EP4.exe" (if you have any shortcut options, insert them here, if not, delete this part of the code)
Save it as "The Sims 2.bat" (or something like that, the file
must end in .bat though), and make sure that "All Files" is selected under "Save as type".