The most common reason for this is that the computer date and time are incorrect. Right click the clock in the system tray and select adjust date/time. Check the month, day, year, and time carefully. Adjust as needed. Reboot once the time has been corrected.
If that does not help, please contact support opr consider trying a reinstall of OneCare on the affected PC before contacting 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