Actually you will no longer be needing any account #
All you will need to activate onecare on another computer is your Live ID. The email address you used to activate your onecare.
Download onecare on the 2nd computer from http://onecare.live.com you can choose to install the 90 day free trial.
Make sure no other anti-virus application is installed before installing onecare.
After installing, onecare will ask you for a subscription, just sign-in with your Live ID. And your onecare on the 2nd machine will use the same subscription as your first computer.