Stop Desktops from Reordering Possible Duplicate:
  Why do full-screen applications change their order in Mission Control?  




Say I'm on desktop 1 and a click an app in the dock for a window on desktop 3.  It will reorder my desktops, moving 3 to 2.  How can I keep it from doing this?
The setting to turn that off is in your Mission Control preference pane.