Start at http://onecare.live.com and select Member Accounts from the left column. In the US, this page is here:
http://onecare.live.com/standard/en-us/members/default.htm
See the instructions for adding a PC.
If you need help, contact support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve