You don't need the product key. Use the Live ID and password originally used to activate your One Care subscription. I suggest uninstalling One Care, use the cleanup tool - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=828910&SiteID=2 then install One Care from here - http://onecare.live.com/standard/en-us/default.htmBefore you reinstall One Care be sure there are no other antivirus or antispyware applications installed on your computer.
If reinstalling does not resolve the issue please contact support. How to reach support - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2