SAN FRANCISCO: Computer security firms reported on Tuesday a new outbreak of the Explorer Zip computer "worm," one of the most damaging computer infections ever seen. The bug was being carried with a different kind of compression software, the computer security firms said, but the effect is the same. The worm can destroy files and data, and the last outbreak earlier this year cost hundreds of millions of dollars damage in thousands of computers around the world.

Because it is a new version of the virus, it has eluded existing anti-virus software, though major firms quickly upgraded programs that combat the bug.

The Network Associates Inc, the world's largest computer security firm, said it had reports from 10 major companies in the high -tech and entertainment businesses that were hit by the e-mail-borne bug.

Dan Schrader, vice-president of new technology at Trend Micro Inc said that three large corporations reported incidents during the day.

The so-called Trojan horse arrives as an e-mail that has the target user's own name on it, and it appears to be from a friend. The recipient is invited to open an attached file that destroys files on the user's disk drive when it is opened.

"It's very insidious -- there's a bit of social engineering going on here," said Schrader.

The Trojan horse "contains a destructive payload," he said, which searches though hard drives and selects a series of files and destroys them by making them zero bytes long. This can make the files unrecoverable.

Schrader said that the way to avoid the virus is to avoid opening unsolicited e-mail attachments and by running current anti -virus software that has been updated for the new infection.