Computer users returning after the New Year break are in danger from the latest mass mailing email worms.

The ZaCker (or Maldal.D) worm can delete antivirus software protection and a variety of critical system files from the PCs of unwary users. Typically, the mass mailing worm arrives by email with the subject line ZaCker and an executable, infected attachment which weighs in at 27KB. It uses Outlook to spread itself to everybody in the Outlook address book. Over the holidays another mass mailer, called Sheer.A (or Zoher) did the rounds. This takes advantage of an IE exploit which means an infected attachment could be opened simply by reading or previewing a message. MessageLabs, a managed services firm which scans its users emails for viruses, reports blocking 8,573 copies of the virus over the holidays. Antivirus vendors advise users to update antiviral protection in order to thwart the spread of both worms.