The first thing I would suggest doing is uninstalling the 2.0 beta. Then try a reinstall after verifying that you have no other antivirus or firewalls installed and running.
As for Defender, it cannot be removed from Vista, but OneCare disables it when OneCare is installed as OneCare includes the antispware protection of Defender.
If you need help with this, contact support - http://help.live.com/help.aspx?project=onecarev2 - use the get more help link in the lower right to contact support.
-steve