The usual reason for this is that you are entering punctuation in one of the fields in the form. You'd think it would be smart enough to point out the problem field, but apparently it simply clears the form and leaves you wondering what's up.
In all boxes - name, address, phone, credit card, etc. you cannot enter anything other than letters of numbers. No punctuation or dashes, etc.
If this is not you problem, you may want to contact customer service -
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
However, I'd suggest having a look at the announcements in the General forum before paying the full retail subscription price at this time.
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator