I'm going to move this thread to the Install and Activate topic folder.
When you reinstalled the OS and installed a fresh copy of OneCare, it is a trial copy until you activate it with the subscription LiveID.
Removing that PC from the Circle from the Hub PC is optional, but a good idea if the PC name may be different after the OS reinstall, for example.
Where you are currently at is a login issue for the LiveID, typically due to network settings.
Have a look at this KB article, particularly method 2:
http://support.microsoft.com/kb/930157/en-us
If that doesn't help, please contact support:
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve