Reinstall or add a computer from http://onecare.live.com - member accounts - add a computer. Activate with your LiveID for your subscription.
As long as the old PC is no longer in use, you are within the terms of the license.
Optionally, make one PC a hub and remove the old PC from your subscription. Open OneCare, click on Your OneCare Circle, make this PC a hub. When the other 2 PCs appears, click the remove link for the retired PC. Once it completes, you can click to stop the PC from being a hub PC.
-steve
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