At a theoretical level, an ISP could set up a slush fund to retain a computer maintenance company not unlike Geek Squad, basically a local company or geek who could do a basic AV/Trojan scan and clean, and perform that service at a reduced price for the zombied customer. That would give the ISP a disconnect from the further responsibility to keep their computer clean. A local company gets more business, the zombied computer gets cleaned up. Of course, there are zombies that require a full reformat/reinstall of the OS, and that would incur a higher cost for the customer. And most ISPs are nation-wide operations, so they'd have really far too much hassle trying to set up local contacts to subcontract such a service to.
Education is the key here. Most people don't know how to provide minimal defenses for their PC. Most of us at SQLServerCentral have our home computer behind a router with A/V and anti-malware software, maybe with Zone Alarm or something else installed, but we're definitely the minority. The retired couple across the street just want to send pictures and email back forth to their kids and grand kids, but once they get rooted, they'll also get pretty confused when you start trying to teach them how to defend their computer.
Solution? Dunno. Look at how many computers get the "Free Anti-Virus" which is a 45-day eval which then goes south, and the people think they're still protected because they've got Norton. I think one step would be better initial configuration from the vendor, possibly requiring them to at least update their install images monthly so that fewer patches have to be downloaded after the PC is purchased. And this could be a behavior required by MS. I know some small PC vendors pre-package live, free A/V like AVG, something like Spybot, but then you once again run into the education issue so that people know the proper way to respond to AV alerts, much less something like alerts from Zone Alarm Pro.
And people wonder why I went to a Mac?
I may not be immune to malware, but I'm certainly much more resistant.
-----Knowledge is of two kinds. We know a subject ourselves or we know where we can find information upon it. --Samuel Johnson