"Nearly indestructible botnet" - Kaspersky

"The TDL-4 botnet is the most sophisticated threat today" says Kaspersky Labs.  It has infected over four million PC's  creating a robot network (botnet) and is pretty much indestructible.  Computer World

Old style botnet with central
command structure.

Suggested by Brian Hall.

Malicious code sneaks into the very first sector of the hard drive, the part which lights up before the operating system is turned on.  TDL-4  uses strong encryption and circulates its evil messages in a loose peer-to-peer network instead of along a rigid pipeline that is easier to police.