You apparently purchased a copy of OneCare at retail that is version 1.5 or 1.6. Version 2.0 is the first version that installs to 64 bit Vista.
Install from the web - http://onecare.live.com and activate with your LiveID, and if this is your first activation, select the option to provide a retail key or token and use the key from inside the retail package to activate your subscription.
Note that if you have not yet activated the subscription on any PC, use OneCare in trial mode for up to 90 days before clicking the "subscribe" link in OneCare to delay starting the paid subscription until your free trial period is up.
-steve