A botnet called TDL has apparently captured/enrolled/enslaved 4.5 million PCs and it may be very difficult to disable. The evil geniuses behind TDL have apparently learned from the shutdown of other botnets and have become adept at hiding the code for TDL in areas of a computer that anti-virus software would never think to look. It's the equivalent of running your drug smuggling operation out of a preschool.
The TDL virus spreads via booby-trapped websites and infects a machine by exploiting unpatched vulnerabilities. The virus has been found lurking on sites offering porn and pirated movies as well as those that let people store video and image files. The virus installs itself in a Windows system file known as the master boot record. This file holds the list of instructions to get a computer started and is a good place to hide because it is rarely scanned by standard anti-virus programs.