Even though no new major viruses were introduced last month, a few steadfast problems continued to plague networks around the world.

The Sobig family of viruses teamed up with Klez and Bugbear to wreak havoc on the Internet, and retain their notorious spots on two separate lists of the most troublesome viruses in the wild for the month of July.

''Forget the Osbournes,'' says Chris Belthoff, a senior security analyst with Sophos, Inc., a security and anti-virus company based in Lynnfield, Mass. ''The Sobig family of computer worms, accounting for more than half of all virus reports, is most likely to strike fear into the hears of IT administrators.''

Sobig and all its variants have posed quite a problem in the last seven months. Five Sobigs have been released this year. Sobig-E, a new variant, accounted for 47.8 percent of all worm and viruses incidents reported to Sophos last month. And it accounted for nearly 18 percent at Central Command, Inc., an anti-virus company that also tracks the worst virus offenders.

Sophos analysts report that, combined, the Sobig worms have had the biggest impact on business networks so far this year.

But while the Sobig family continues to plague IT managers, Klez and Bugbear are holding onto their top rankings as well.

Central Command ranks Klez as the worst offender, accounting for 19.2 percent of reports. Sophos put Klez in the third spot, with 5.9 percent. But Klez is unique in its staying power. It has remained on the Sophos list for the past 18 months in a row.

''After relinquishing the top spot over the past couple of months, Klez-E once again regains the peak position,'' says Steven Sundermeier, product manager at Central Command. ''What we have seen with Klez-E is unparalleled to any past Internet worm, as it continues to show extraordinary staying power.''

Central Command also reports that Klez is now noted as the most prevalent virus on record.

''Fortunately, July 2003 saw no new significant virus outbreaks,'' reports Central Command analysts. ''The only minor noise produced was for the Worm/Gruel family. However, several attempts by the virus author to proliferate its creation proved unsuccessful.''