OneCare will disable Defender. If you are seeing it in services, that's okay - leave it set to automatic. The listing in msconfig should be left as is, the interface for Defender will not start on boot.
If you have the Defender icon running in the System Tray, that would be unusual in that OneCare should have prevented it from loading.
If you open OneCare, change settings, and go to the Tune-up tab ad "Change Startup Settings" you will see Defender listed as enabled, but you can't change it to disabled there yourself.
-steve