You will not get a key.
You download the trial from
http://onecare.live.com and once it is installed, click subscribe in OneCare and sign in with the same LiveID you used during the purchase to activate with your subscription.
For more information see Instant Help - http://help.live.com/help.aspx?project=onecarev2&mkt=en-us&querytype=topic&query=OneCare_PROC_FindLiveIDHelp.htm
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator