I'm going to assume that you have a copy of OneCare that is a 1.0 or 1.5 retail CD. If it is 1.0, it won't run under Vista at all. If it is 1.5, it will not run on Vista x64.
Since the retail CD is simply an installer that goes online to get the latest version and supporting files, simply go to http://onecare.live.com and install from there. Once installed, activate with your LiveID and the retail product key from inside the box if this is your first activation.
-steve