Run the cleanup tool:
Cleanup tool - http://social.microsoft.com/forums/en-US/onecareinstallandactivate/thread/399071e2-49dc-4767-932d-d261e3965943/
Then install the trial from http://onecare.live.com and activate/subscribe with the LiveID for your subscription to get going again.
It sounds like you didn't actually reinstall Windows, but installed it on top of itself. Doing so causes installed programs to break in many cases. I suggest going to Windows Update to get all available updates, too.
If you are unable to get OneCare reinstalled, please contact support for help.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator