If you can't find an obvious solution to a network problem, it never hurts to run a complete scan for viruses, worms, Trojan horses, and spyware on each computer connected to your LAN. Even if you have firewalls, up-to-date antivirus programs, and other network security software running on all your computers, it's possible that something might have slipped through your defenses.

Several antivirus program vendors offer free online scans that might identify a virus that your resident program might not catch. As part of your troubleshooting routine, run a full scan with your usual network security programs and also use one or more of these online scans:

Use an online scanner made by a different supplier from the one that came with the antivirus program resident in your computers. Each company employs a slightly different set of rules for finding and isolating viruses, so you will want to take advantage of more than one approach.