S7777, about that sword.
McAfee have made changes to their VirusScan User Interface with each major version change. I've recently reverted to Ver 5.21 (which I bought on-line) from Ver 7 (which I bought in a box) because Ver 7 doesn't sit happily with Win 2K.
With Ver 5.21 there is a desktop icon labelled "VirusScan Central" and this leads you to the Interface where you configure the three main functions, System, Download and Internet Scan. (There is actually a fourth called E-mail Scan but you have no control over that because it is automatically configured when you configure Download Scan. It displays "Disabled" permanently which is very confusing and disturbing until you find out that it is OK.)
The icon for System Scan is the Shield with the sword. The icon on the Systray of my Task Bar doesn't have a sword no matter what I do so I don't know what it is supposed to tell you.
Check the exclusions tab for each of the configurable options. It is possible that a virus/trojan has changed the exclusions options and that that is why you are not getting virus alerts.
Only my System Scan has an exclusions option and that was set on installation to the default of "/Recycled/".