Why does Xcode open when I start OS X?

Solution 1:

You can see everything that is supposed to open on login in System Preferences > Users & Groups > Login Items. These will open even if you uncheck "Reopen windows when logging back in." in the logout dialog box.

Solution 2:

Open System Preferences -> Click on Users & Groups -> Choose your user and click on the Login Items tab.

Is the file (libwpg.rb) in the list?

If so, select libwpg.rb and then click the minus button to remove it from the login items.

Solution 3:

you sound like you know what you are doing - so I may be way off, but... Have you checked that these two options are not ticked when you right click the xcode icon and look at Options?