One of the recently reported malware exploits actually does contain an AV program that scans your machine to remove all other malware of similar kind.

The bot programs that allow someone to take over your computer to send SPAM, to launch DDOS attacks, and;or just to steal everything you've got are becoming common enough that they often compete with each other, so these "authors" hacked an AV program from a major AV supplier and incorporated it into their "bot" so that they'd have exclusive use of your machine.

At the time of the last report I saw, it was still undetermined what use the people distributing this intend to make of the machines they manage to infect; but it isn't likely something you want your machine to participate in.

It's fairly common for malware to try to kill your AV and other anti-malware stuff, but these guys took a sort of novel approach on it.

John