Install OneCare from
http://onecare.live.com by choosing the trial. Activate your subscription with the retail key and your LiveID or just the LiveID if this is an install on another PC for an existing subscription.
OneCare will install on 32 bit XP SP2 or later and Vista 32 bit or 64 bit. However, older installation CDs for OneCare may not run on Vista, or even XP SP3.
-steve
Microsoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator