Late last year, I posted "Hackers Shifting Targets", a discussion about hacker shifting their focus to attack virus and malware scanners in order to propagate their schemes.
Thanks to Boing Boing for posting "Malware kills competitors with anti-virus warez", describing a real-life version of scenario.
SpamThru installs it's own anti-virus software which, naturally, avoids removing itself. It does remove the rest of the viruses and malware it detects, leaving itself 'in charge' of the system.