WASHINGTON--Considering the serious warnings lately about Internet viruses, users may be encouraged to unplug their machines just to keep them safe. In reality, common sense and a few basic tips can keep Internet surfers secure. Antivirus software is a must, and it should be updated at least once a week to recognize newly released viruses. People with broadband Internet connections should also have a firewall. These protect the computer from attacks by hackers. Then it is time to think about computer habits. Unlike old viruses, which spread only through shared diskettes, the Internet brings viruses straight to the email-box. If email with attachment comes from an unknown sender, or unexpectedly from a known sender, it should not be opened. On receipt of an unexpected attachment from a friend, experts suggest a telephone call to make sure of the mail's source before opening. Computer viruses cannot spread through   _  .
shared diskettes shared floppies email telephone lines