Sorry, I can't help but laugh at this. $500 are not "trash" computers, those are >budget< computers for people, well, less fortunate than you. In fact, a $500 custom made PC can have way better components than the cheaper Mac Mini available.
Viruses don't get into Windows without user interaction. Those days are over. I won't lie and say that the NT kernel is as secure as any *nix kernel out there, but security is something MS has worked very hard to get right.
Viruses on Windows, people bitch with good reason. MS adds UAC and people bitch. MS improves UAC and a few still bitch. What gives. The UAC interaction principle is the same as in any other system: "Application wants access to system files?"---> "Ask user for permission". Remember the trojan that was being spread on pirated copies of iWork? Well, you ARE safe as long as you don't allow risky **** in your computer. Common Sense.