You may also need to verify that you are not including any punctuation in your submission form for the payment information. No dashes, no commas, no periods, etc. Just letters and numbers.
If you are unable to complete purchase, please contact customer service/support.
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
Before you take the plunge and purchase, you may want to look here:
http://social.microsoft.com/Forums/en-US/onecaregeneral/thread/93ef4afe-d676-4e5f-bbc8-c9ef06295898
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator