You can't change the ID used for your subscription. On the first PC, open OneCare and click Help and then About to see your Subscription ID. That is the LiveID to sign in with.
If the activation dialog already contains a LiveID from a prior attempt or one used in trial mode, but this is not the one you need to activate with to associate it with your subscription, see this post for how to reset this - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2776450&SiteID=2
If you don't know the password for that subscription LiveID, or the subscription LiveID used by Circuit City is not shown in Help/About, you can contact support to identify and reset the LiveID if you have the retail key from the original installation.
Circuit City should have given you a paper with the LiveID and password used for the subscription setup.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve