Download and run the Cleanup Tool for OneCare:
http://social.microsoft.com/forums/en-US/onecareinstallandactivate/thread/399071e2-49dc-4767-932d-d261e3965943/
After it has completed, reboot and try installing MSE again (since that is what you were trying to do anyway).
Install from the official MSE download page.
If that fails to work, please open a support case.
To open an email support case for MSE, start here:
https://support.microsoftsecurityessentials.com/
You can also pursue a support case with OneCare support -
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 ~