Again why? Most computer users want to surf the web, get email, and do a few other things like word processing. Unless you are totally a game nerd, then a Mac can do those better and safer. More and more games are coming for the Mac as well.
As to cost, save the $150, apply it to a Mac Mini... and not worry about spyware, viruses, worms, pop-ups and other mal-ware.
Even if a person only wants to do internet and word processing, a PC is cheaper, and protecting it is no big deal. A friend of mine just bought a Dell. He hooked it up to a router, and it had MacAfee security suite already installed. He's had zero problems.