The thing about windows viruses for the most part is that they usually come in an executable that the user installs either on purpose (new toolbar, cool free game, etc), or accidentally through some form of trick. Sometimes they come in the form of infected PowerPoint files or something like that, but Microsoft is getting pretty good at stopping those guys. The bad guys finally figured out that their best ally is the user who wants something installed. So they hide in fake toolbars, etc. You may not even know you are installing a virus, and if you click through all of the wizard dialogs to install something, there's not much a virus scanner can do until it is too late. Hopefully it will notice things are bad right after you install it, but these days, the first thing the smart viruses do is disable antivirus software.
I think the oddest thing about this one is that it does so much damage. That's not usually what they do these days. It's much more profitable to set your computer up as a zombie and become part of a botnet to send spam, steal identities (including yours), etc. The fact that it destroys files is surprising.
Wade, do you have any network shares on your computer? My guess is that there was one point of entry, and the rest of the computers fell to a worm attack by the first computer infecting files on the network.
Mac users tend to feel like they don't need antivirus software, and to a degree it is true that they are less likely to be hit by viruses. But that's only because people don't write viruses for macs often. That will change as more people uses macs. There have been a few mac viruses in the wild, and Apple recommends antivirus software be used. Apple took a fairly secure base in the mach kernel and hamstrung the security in the name of user friendliness. The macs are always the first to go down at the hacker competitions.
I was using Kaspersky on the other 2 which is a top rated software.
