After you've set up the subscription initially with a LiveID when signed in on one Windows account, and you then sign on with a different Windows account and are asked to activate OneCare, simply provide the same LiveID and password to activate under that login. Actually, I don't think you should have to activate under the additional Windows accounts, but it has been reported to be required.
Note that activating on the same PC with a different Windows account and the same LiveID for your subscription does not count as another installation for your 3 licenses.
-steve