Techdirt provides A Look Into How Spammers Spam:
some anti-spammers have managed to get themselves into the various “spam clubs” where top spammers trade tips with each other. From that they’ve learned, as we all pretty much knew already, that the state of the art in spamming is using compromised machines that have had a trojan installed on them. Lists of such controllable machines are offered to other spammers, both for additional spamming capabilities and for denial of service attacks. Basically, the spammers have built up quite a distributed super computer – and those contributing cycles and bandwidth don’t even know about it.”
Ed Bott 