"Malware today operates very differently from when the rootkit craze was going on," he said. "Then... malware would throw popups all over your desktop and take over your browser. Today we're seeing a totally different type of malware."

Today's malware runs quietly in the background, spamming or hosting its nasty Web sites without the victim ever noticing what 's going on. Ironically, though they are built to evade detection, the most sophisticated kernel-level rootkits are often so incredibly intrusive that they draw attention to themselves, security experts say.

"It's extremely difficult to write code for your kernel that doesn't crash your computer," said Alfred Huger, vice president of Symantec's Security Response team. "Your software can step on somebody else's pretty easily."

Huger agrees that while rootkits are still a problem for Unix users, they're not widespread on Windows PCs.

Rootkits make up far less than 1 percent of all the attempted infections that Symantec tracks these days. As for Rustock.C, despite all its technical sophistication, Symantec has only spotted it in the wild about 300 times.

"On the whole malware spectrum, it's a very small piece and it's of limited risk today," Huger said.

Not everyone agrees with Symantec's findings, however. Thierry Zoller, director of product security with n.runs, says that Rustock.C was widely distributed via the notorious Russian Business Network and that infections are most likely in the tens of thousands.

"Rootkits were used to hold access to a compromised target as long as possible and never had the goal to be spread widely," he said in an interview conducted via instant message.

In the end, criminals may be avoiding rootkits for a very simple reason: They just don't need them.

Instead of using sneaky rootkit techniques, hackers have instead developed new techniques for making it hard for antivirus vendors to tell the difference between their software and legitimate programs. For example, they make thousands of different versions of one malicious program, jumbling up the code each time so that antivirus products have a hard time spotting it.

In the second half of 2007, for example, Symantec tracked nearly half a million new types of malicious code, up 136 percent from the first half of the year. Security experts say that this situation is even worse in 2008.

"The stuff that we run across is not that complicated," said Greg Hoglund, CEO of HBGary, a company that sells software to help customers respond to computer intrusions. "Most of the malware that's out there nowadays... doesn't even attempt to hide."