Spyware authors are prepared to pay botnet farmers or webmasters much more for infecting PCs in the UK or Australia than machines in continental Europe.
Selling "installs" is a common practice in the cyber-underworld, the most notable example being in 2005 when Jeanson Ancheta was arrested for building a 400,000-strong botnet and installing adware from 180 solutions for a fee of $60,000. Cybercriminals have since moved on to installing spyware onto compromised machines.
Zombie machines infected with Trojan horse malware can be used to relay spam or launch denial of service attacks. Compromised machines can be also be pointed to websites from which additional items of malware can be downloaded. The practice is normally used to update Trojan code, but it also creates a means for cybercrooks to make a "nice little earner".