OneCare will always turn off the Windows firewall as OneCare includes its own firewall. As for the problem that you are decribing regarding the Windows Security Center - see this thread:
http://social.microsoft.com/Forums/en-US/onecareupdate/thread/297455ff-0484-4dd6-9627-ca1b0e30b015Your solution is to either disable the Security Center alerting or update Vista to SP2.
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator