Unwanted App Launch Problem Solved
There can be a file in your top-level Library's Preferences folder called com.apple.SystemLoginItems.plist. This mechanism is used by a very few specialized applications that launch on a per-user basis and want access to the Window Server, but need to launch before the user actually logs in. (Timbuktu is an example of such an application.) It is deprecated in Leopard, where you're supposed to use a LaunchAgent instead; but some older programs still use it.
