Location of "start at login" items?
The following list might help you to disable some program at startup :


Those in System Preferences » Accounts » Login Items (actually, these include the ones from 1.)
The login hook for loginwindow described here, read current value with defaults read com.apple.loginwindow LoginHook
Some of those specified as part of a launchd script (or otherwise managed by launchd). List all with sudo launchctl list. See man launchctl for details.


Regarding the launchd lists, you can see their definition files in the LaunchAgents and LaunchDaemons subdirectories of your system's Library directories:


/System/Library (you probably shouldn't touch these)
/Library/ (usually for all users)
/Users/yourname/Library (your own programs)


In the latter two libraries, check for anything suspicious. It's usually XML, so you can open and read them.