If one cannot afford to Upgrade a Windows Computer, how do you expect them to purchase a Apple? Tat they are invulnerable to attacks is a myth. Did the not just recently issue security updates? On the other hand you can buy W-7 Computers fairly cheap. I paid for a W-7 with 500 GB Drive and all the bells and whistles for $450.00 that was for a 18.4 Full HD 1080p LED LCD. intel core i3-380M granted it is a Notebook.
On the other hand my Desktop was just $325.00 same features except that I already had a 23" Monitor. You can buy both for the price of one Mac.