Check the PC date and time. Correct it if it is wrong - day, date, month, hour, minutes.
Reboot.
If that doesn't resolve the problem, please contact support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
What other programs also won't run? What is the exact error reported by OneCare when you try to open it? Do you get any errors before trying to open OneCare? I assume that the system tray OneCare icon is red.
-steve
Microsoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator