Since you are installing a fresh copy of OneCare with a CD and going to the Billing page for your LiveID doesn't show the subscription (which means your subscription was under a different LiveID), you should treat this as a new install. Using the LiveID you didn't see the subscription with at the Billing site, proceed with activation, but I suspect that whatever other antivirus product you installed has not been completely removed.
Use the Cleanup tool - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=828910&SiteID=2 - to remove all of OneCare from the PC.
Remove the other security software completely, using a utility for that program if needed and available from the vendor:
Norton Removal Tool - ftp://ftp.symantec.com/public/english_us_canada/removal_tools/Norton_Removal_Tool.exe
McAfee Removal Tool - http://www.majorgeeks.com/McAfee_Consumer_Product_Removal_Tool_d5420.html
Review this post for the KB article to troubleshoot install problems - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=1588602&SiteID=2
Then install from http://onecare.live.com and activate with your LiveID and PIN.
If it still doesn't work -
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve
