National policy appears to have an impact on botnet populations. Countries that have joined the London Action Plan--an effort to coordinate anti-spam and anti-cybercrime efforts internationally--or who are signatories of the Council of Europe's Convention on Cybercrime have fewer botnet infection, the researchers say.

While there is a relationship between the size of an ISP and the number of infected machines connected to the Internet through the provider's network, some providers have 100 times more infections than others of the same size. And while some ISPs are addressing the problem, most are failing to meet the magnitude of the issue, Delft University's van Eeten says. One large ISP recently removed 1,000 infected systems a month from its network, but it likely had 40,000 to 200,000 compromised computers connected to its network."There is no way for a consumer to assess any of the claims that a particular ISP cares about its security," van Eeten says.

The researchers hope to change that by developing metrics that show how actively an ISP is detecting and mitigating compromised systems, and they're working with the Dutch government to develop such metrics.

Requiring ISPs to secure their customers' computers would have mixed economic impacts, according to experts. Blocking consumers from connecting to the network if their computer is compromised would lead to an avalanche of costly support calls, according to Jose Nazario, senior researcher at network security firm Arbor Networks.

Still, Internet service providers are taking the threat more seriously. Providers in Australia, for example, have signed an agreement to notify consumers if their PC is compromised by malicious software and to possibly downshift their bandwidth. In the Netherlands, 14 ISPs have agreed to exchange information about security issues, to notify users if their system appears to be compromised, and to block traffic from infected systems, essentially quarantining users.

While the research suggests that focusing on the dozens of network providers with the largest botnet populations connected to their networks, Trend Micro's Rand stresses that the entire problem has to be addressed more holistically."Everyone needs to deal with the problem simultaneously," Rand says. "We could fix the top 50 ISPs this year, and next year, we'll find we are dealing with the top 500."