Run the Cleanup tool - http://social.microsoft.com/forums/en-US/onecareinstallandactivate/thread/399071e2-49dc-4767-932d-d261e3965943/
If that fails, contact support -
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
Personally, I'd not bother reinstalling OneCare.
See this post for End of Life information for Windows Live OneCare - http://social.microsoft.com/Forums/en-US/onecaregeneral/thread/81996436-edad-41ba-bdfc-02fc094e2ca9
Uninstall OneCare http://social.microsoft.com/Forums/en-US/onecaregeneral/thread/732e6371-2e71-479a-8cd6-b189941ed4fb, use the cleanup tool: http://social.microsoft.com/forums/en-US/onecareinstallandactivate/thread/399071e2-49dc-4767-932d-d261e3965943/, and then download and install the free Microsoft Security essentials for malware protection - http://www.microsoft.com/security_essentials/resources.aspx
If you used OneCare for backups, you'll still be able to restore from old backups if you use the OneCare Restore tool - http://social.microsoft.com/Forums/en-US/onecarebackupandrestore/thread/e5d686c8-762b-4b22-bc08-bfb93f911e57, but you'll need to seek an alternative backup solution.
-steve