The first thing I'd suggest doing is checking your PC date and time settings by clicking on the clock in the system tray. Make sure that they are correct. If it is very wrong, you can encounter a problem with OneCare.
If you can provide a bit more information about the exact error message, as Wenske has requested, that would be helpful. Also:
Was OneCare working fine before?
Did anything change since the last time it worked?
What operating system are you using?
Have you tried rebooting the PC?
If your date and time are incorrect, fix them and reboot.
You can also contact OneCare support -
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve