The 2100 error can mean a few different things, but the most common reasons are that there is a problem with your Billing profile at http://billing.microsoft.com in that the country doesn't match your location or address information for the payment type or that there is actually a problem with the validation of your credit card information.
If the problem persists, please contact support -
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
Alternatively, instead of subscribing with a credit card, pick up a retail copy and use the retail key to subscribe.
-steve