Make sure that the McAfee VirusScan software has been properly installed on your computer and that no other antivirus programs are running.
Right-click on the shield icon in the system tray at the bottom of the screen and select "VirusScan Console."
Double-click on the "On-Access Scanner" entry in the list of antivirus tasks.
Click on the "General Settings" tab on the left side of the pop-up window.
Go to the "General" tab at the top of the settings window.
Check the box next to "Enable on-access scanning at system startup."
Click "OK" to save the settings. Restart your computer to have the OAS feature start running.