To reinstall One Care first run the install cleanup tool -
http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/399071e2-49dc-4767-932d-d261e3965943 then install the trial -
http://onecare.live.com/standard/en-us/install/install.htm To activate, open One Care, select "Subscribe" and when asked for a Live ID enter the Live ID and password associated with your subscription and One Care will activate.
Jim - MVP Windows Live - Forum Moderator - Live One Care - Live Mesh