Saw this on BBC

More than four million PCs have been enrolled in a botnet security experts say is almost "indestructible".

The botnet, known as TDL, targets Windows PCs and is difficult to detect and shut down.

Code that hijacks a PC hides in places security software rarely looks and the botnet is controlled using custom-made encryption.

Security researchers said recent botnet shutdowns had made TDL's controllers harden it against investigation.

The 4.5 million PCs have become victims over the last three months following the appearance of the fourth version of the TDL virus.