I'm going to assume that your question is off topic for the OneCare forum -
http://onecare.live.com Any software that requires activation with a product key typically ties that activation with a hardware hash to the key. So, if you install and activate, but then make a major hardware change, you may need to activate again as it will invalidate the existing activation.
If your key allows you to activate on a single PC, trying to activate on another PC while the first is still in use is a violation of the license in most cases, even if you are able to proceed.
If you are not off topic, and are referring to OneCare activation, you install and activate with a key once. Subsequent activations, up to 3 computers with OneCare running and active, take place using the LiveID you used for the first activation.
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator