At some point in the installation process for a second system I entered the wrong (but valid) Live ID. Now when I try to subscribe, the Live ID field is filled in and can not be changed to a different Live ID.
How can change the Live ID that it attempts to subscribe to?
Unfortunately, you've encountered a bug that will require you to uninstall OneCare and reinstall it from http://onecare.live.com to enter a different LiveID to activate.