You don't install javascript, it is either allowed or blocked in your browser settings. If you are using any browser other than Internet Explorer 7, change the default browser to IE under Program Access and Defaults in Control Panel. If you are using IE7, go to tools/Internet Options/Advanced, and choose the option to reset all settings to default.
Try to activate again after that.
If you continue to have problems, please contact support.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve