Code Red II's back doors into computers harder to get rid of.(Knight Ridder Newspapers)

An offshoot of the Code Red worm has started spreading across the Internet, disrupting some operations at The Associated Press, Motorola and other companies, and causing some network outages. The more malicious Code Red II, which was discovered Saturday, infects computer servers and leaves behind a so-called back door that allows hackers to gain control of infected systems.

Applying Microsoft's patch and rebooting the infected computer does not get rid of the Code Red II worm. The only real solution is to reformat the system's hard drive and reinstall all software, security experts say.

"What I know is that with the new worm, if you patch your system, you will not get rid of the back door," said Alan Paller, research director at the SANS Institute, a computer security think tank …