Typically, the Circuit City techs create a LiveID for you and include it and the password on a form they give you at delivery. If you've lost that form, or the tech who set you up failed to give you one, you may be able to get the ID reset by contacting support. You can find the ID by opening OneCare and clicking Help/About...in the upper right hand corner. The subscription ID is the LiveID. If the subscription ID is not shown, the LiveID can be identified by support with the key you have from the retail CD.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
Alternatively, purchase another retail copy of OneCare and uninstall/reinstall (from http://onecare.live.com) and activate with your own LiveID and the retail key. You may actually save money this way as an online renewal is typically full retail price.
-steve