Set your default web browser to be Internet Explorer. Open IE, go to Tools, Options, and clear all cookies and temporary Internet files.
Close Internet Explorer.
Open OneCare and click on subscribe.
Sign in with your LiveID. (If you have never created a LiveID and don't know what that is, go to http://account.live.com and create a LiveID)
Proceed with Activation.
If that still fails, please contact support. Select the option that you are a trial or beta user to proceed to the email support form without validation, since you do not yet have a subscription.
http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve