Disable alt-tab (option-tab) instead of command-tab
It sounds like you've managed to change your keyboard mappings.  Assuming you haven't installed 3rd party mapping software, the quickest fix is to restore the mappings back to default.

SystemPreferences -> Keyboard -> Keyboard Shortcuts -> Keyboard &amp; Text Input -> Restore Defaults

Most likely one of the "Move focus to X" options is set to alt-tab