You add your second PC by going to
http://onecare.live.com and selecting Member Accounts on the left, then add a computer on the right, following instructions to download, install and activate with your subscription Live ID (found under Help/About in OneCare on the 1st PC).
If you are encountering a error:
See this post for the KB article to troubleshoot install problems - http://social.microsoft.com/forums/en-US/onecareinstallandactivate/thread/ba26b781-da18-4ce4-be8c-22647bd5d8fc/
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90 - do this on the PC that is already activated so that you can see your support options including chat and phone as a subscriber.
-steve
~ Microsoft MVP Windows Live ~ Windows Live OneCare| Live Mesh|MS Security Essentials Forums Moderator ~
