Unfortunately it seems that new viruses are being released just about everyday, so the first thing you need to check is that the virus definitions are current both on the server and every workstation on the network.

You also need to ensure that 'Background scanning' or 'Auto-Protect' is enabled on all computers, which means that your anti-virus software is monitoring all program activity on the computer, and will intervene if virus activity is detected. (You may wonder why anti-virus software would ever need to be disabled? When installing new software it is often recommended that anti-virus software be disabled so that it's screening doesn't interfere with the installation process).

Another item to check is that all security updates have been installed on each computer on the network.

Finally, what approach is being used to detect e-mail viruses? Is one anti-virus product being used for all virus screening, or is a separate product also being used for incoming e-mail (at the server)? With the prevalence of destructive e-mail "worm" viruses in recent times, a separate screening program is preferred for e-mail as it means that you have two sets of virus definitions from two different anti-virus vendors scanning for hostile payloads.