It may be a hack causing the problem. Run the Sims 2 Hack Conflict Detection Utility:
http://www.simwardrobe.com/ You can also try deleting the groups.cache file from mydocuments/eagames/thesims2.
If all else fails, drag your Downloads folder to your desktop then run the game. If it runs fine without the Downloads folder, then you know you've got a bad file or files. If that's the case, you can start replacing your downloads a few at a time until you find the culprit. Or you could use the Clean Installer, have it scan your downloads, then sort by date and start disabling downloads individually starting with the most recent.