A new and improved botnet that has infected more than four million PCs is "practically indestructible," security researchers say.

"TDL-4," the name for both the bot Trojan that infects machines and the ensuing collection of compromised computers, is "the most sophisticated threat today," said Kaspersky Labs researcher Sergey Golovanov in a detailed analysis Monday.

"[TDL-4] is practically indestructible," Golovanov said.

Others agree.

"I wouldn't say it's perfectly indestructible, but it is pretty much indestructible," said Joe Stewart, director of malware research at Dell SecureWorks and an internationally-known botnet expert, in an interview today. "It does a very good job of maintaining itself."

Golovanov and Stewart based their judgments on a variety of TDL-4's traits, all which make it an extremely tough character to detect, delete, suppress or eradicate.

For one thing, said Golovanov, TDL-4 infects the MBR, or master boot record, of the PC with a rootkit -- malware that hides by subverting the operating system. The master boot record is the first sector -- sector 0 -- of the hard drive, where code is stored to bootstrap the operating system after the computer's BIOS does its start-up checks.

Because TDL-4 installs its rootkit on the MBR, it is invisible to both the operating system and more, importantly, security software designed to sniff out malicious code.

But that's not TDL-4's secret weapon...