Open the retail box, and find the retail product key. Install OneCare from the web - http://onecare.live.com - for your x64 Vista machine as the 1.5 CD will not install to Vista x64. Once installed, let it run in trial mode for up to 90 days. When you are ready to activate, use the PIN - the retail product key - from inside the 1.5 package to activate your subscription. Your subscription for 12 months begins the day you activate with the key, so the trial period allows for some free usage time.
-steve