Most experts agree that the best place to run antivirus software is on the desktop because all malicious code—no matter how it arrives—eventually does its damage on the desktop (PDAs and other wireless devices excepted). Whether the malicious software comes from the Internet, an email message, an Instant Messaging (IM) session, or a disk, a desktop scanner should catch it. If you can afford to protect two separate locations, also run the software at the email or Internet gateway to get at the source of most new malware. A multitiered defense is important because failure at one level is highly likely sooner or later. If something gets past one level of protection, the presence of another level is your only line of defense.
