To clarify Jim's correct answer - installation of OneCare on Mac OSX is not supported. OneCare can be installed on 32 bit XP SP2 or SP3 and 32 bit or 64 bit Vista only. If the Mac is running XP or Vista, OneCare will install. If it is running OSX, it will not.
-steve
Microsoft MVP Windows Live / Windows Live OneCare & Live Mesh Forum Moderator