being unix based, the user allows every program downloaded to run/install by authorizing it with an administrator's password. with windows, this is not the case, a program can be hidden in another file, then executed/installed to run without the user's notification. that is the main reason macs don't get viruses, there are mac viruses, but you have to try to get them...
PS- I did a large school report on this, PC majority is a minor reason, but still legitimate