It won't activate the subscription and only charges one dollar to my Mastercard account instead of the $49.95 for the one year subscription.
I don't know why it is failing to complete the transaction or why there is a $1 charge, but I suggest contacting support -
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve