Several Irish businesses were still suffering the effects yesterday of the latest computer virus to wreak havoc on computer systems.

Microsoft's operation at Sandyford in Dublin is believed to have been particularly badly affected. It is understood that a six gigabyte drive, containing enough information to fill a computer server, had all its files deleted by the malicious "worm". The worm burrows into files stored on an individual's computer drive or a network of machines connected by a server. A Microsoft spokesman was not available to comment.

The ExploreZip worm, thought to have originated in Israel early last week, enters computers as an email attachment and once activated deletes all Microsoft Word, Powerpoint and Excel files on the hard drive. It spreads by entering email in-boxes and attaching itself to automatically generated responses to senders of unread email.

It has emerged that ExploreZip also has the capability to obliterate shared files on a local network. It propagates itself automatically to other networked machines. Where an email is sent to a personal computer linked to a network, ExploreZip can delete all the files on that computer's hard drive and sometimes files on others in a network.

Dublin based providers of anti-virus software said they had had several queries in response to the ExploreZip worm, and they estimated it had generated huge costs in the restoration of back-up files alone. However, they said most organisations with critical information stored on their servers tended to have back-up files.

In one case, an organisation lost some valuable information from a four gigabyte hard drive. But faced by a bill of around £2,000 to retrieve the data, it opted against recovering it.

The ExploreZip worm did most of its damage on Thursday and Friday and is reported to have infected computer systems at several big US corporations.