expert warned yesterday.

But he assured firms: "Don't worry, it's not terminal.'' The bug has cropped up in two companies visited by the computer firm ACT this week.

Software technician Mr. Dennis Pekus said it originated in Switzerland in 1992 and was known as Forms.

Although relatively harmless, it could make an obscene message appear, apparently referring to a woman who angered the bug's creator.

"It can also make your machine make strange noises -- grinding sounds or whining sounds.'' In one company, the bug was discovered during a routine check in a machine brought in from Belgium, Mr. Pekus said. It was soon eradicated.

Most "infections'' were caused by people passing around computer disks, he said. Pirated games could also be a source of viruses.

Anyone doing business with Switzerland should be particularly careful, he added. "If your machine's making noises and giving obscene messages, call a local company and get them to screen for the virus.''