May 3 (Bloomberg) -- A computer worm called Sasser may have infected hundreds of thousands of computers through the Internet and is still spreading, possibly disrupting business today, a security software expert said.

The worm, which is different than a virus because it doesn't need to be attached to an e-mail to spread, causes a computer to shut down and then reboot several times, apparently without causing any permanent damage, said Mikko Hyppoenen, director of virus research with Helsinki-based F-Secure Oyj. The worm was detected Saturday at 4 a.m. Finnish time, he said.

Sasser exploits a flaw in Microsoft Corp.'s Windows 2000 and Windows XP, the two most common operating systems, Hyppoenen said. Increased network traffic generated by the worm, which is similar to the Blaster worm that spread last August, might slow other systems as well, Hyppoenen said.

``Sasser is spreading at an increasing pace,'' Hyppoenen said in an interview. ``Still, the situation is not as bad as we feared during the weekend. People and companies have learnt to protect themselves since Blaster.''

Hyppoenen said some ``large, global companies'' have already reported to him that their entire systems have been infected. He declined to name them. Panda Anti-Virus Software Ltd. estimates that 3 percent of the world's 600 million computers, or 18 million machines, have been infected, Agence France-Presse reported earlier.

Blaster infected hundreds of thousands of computer systems in August, causing problems to cash machines and train and air travel systems. It prompted the Federal Reserve Bank of Atlanta to shut down its computer system and caused a network overload at Sweden's TeliaSonera AB, the Nordic region's largest phone company.

To contact the reporter on this story: Ville Heiskanen in Helsinki vheiskanen@bloomberg.net

Last Updated: May 3, 2004 03:11 EDT