I think there needs to be, either at the ISP level or higher simple tools that people can use to see if their machine is sending out SPAM. I can't be that difficult for an ISP to profile its users by the amount of traffic on port 25 (or maybe secure SMTP, which I can't remember the number for), and then maybe look a little more closely at the people who are sending unreasonably large amounts of traffic.
Presumably at the higher level, people do notice the IP addresses where much of this traffic is coming from, and there could be a web site like, amIazombie.com or something, which looks at your IP address (in the same way bandwidth checkers do), compares it with a list of known bots/zombies/whatever you want to call them and says yes or no.
Or am I wandering around with my eyes closed and these already exist?
Tux, 'cause there's less chance of a Linux machine being got :-)