Something's definitely wrong with your Mac OS X install, since Cmd+Tab switching can't be disabled. Here's some suggestions to try:
1. Repair Permissions (open Disk Utility, select HD, click Repair Permissions)
2. Log in as a different user (or make a new account if you don't have any other users set up) and try it there. If it works, the issue is specific to something in your user account. If it doesn't work, something is probably messed up in the /Library folder, since that folder applies to all users.