Hi Cate,
When in the admin account, go to the system folder and move the multiple user startup from the extensions folder to the disabled extensions folder. Also, move the multiple users control panel to the disabled control panel folder...
It could be that your prefs somehow got damaged... after everything is disabled, try trashing the preferences for multiple users (i believe there are three in the preferences folder), restart the computer, and then in extensions manager, re-enable multiple users, restart, and then go to the multiple users control panel (the caveat here if you do this is that you will need to redo your user accounts because you have trashed them)....
Multiple users, itself could have gotten damaged, and if that is the case, it might be best to reinstall it from the cd...
good luck

dave