F-Prot's got one of the most generous licenses I know of. Their "home use" product says $29.95 to cover FIVE computers for up to a year. This is good, no, excellent considering I have more than one computer around. In fact I'm using 3-4 of my 5 eligible licenses at the moment and I think it is most certainly worth the money.
They used to be really big in the DOS days, not as famous now, but they sure have kept up. Their product is quite effective, it's nailed a few nasty buggers before they've had a chance to take me down.
Plus, their UNIX versions are free for personal use!
PS: the worm is exploiting an operating system loophole, not the lack of antivirus per se. Won't get into the specifics of why. The three pronged defense is always patch your system, always run antivirus AND antispyware, and keep your firewall up. That way if one of them fails, chances are the other two may slow it down enough to buy you time.