Try the following:
Set IE to be your default web browser (In XP, Set Program Access and Defaults; In Vista, "Default Programs")
In IE7, go to Tools/Internet Options/Advanced and click the option to reset IE to defaults.
Try to complete the installation/activation.
If that doesn't work, please contact support: (via email since your subscription hasn't yet been created)
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve