Just use Sophos on the the Mac and MS Security Essentials on Windows and you'll be fine.
Not wanting to be arrogant or rude, but the simplest way to answer your other questions is that they don't make any sense because they make erroneous assumptions (the not so simple way as to why they are absurd questions would imply explaining technically how Virtual Machines work, and I don't have the patience for that, and I sincerely doubt anyone else on these forum will explain correctly these questions).
Last edited: Oct 31, 2012