How to disable or enable on-access scanning in McAfee Virus Scan
By default, the copy of Mcafee Virus Scan available comes configured to prevent Virus Scan services from being stopped. This is a safeguard against viruses stopping McAfee services and disabling Virus Scan. However, this also causes the 'Disable On-Access Scan' menu item to be grayed out when right-clicking on the 'VShield' icon in the system tray.
The solution to the problem
Follow the instructions to disable or enable on-access scanning in McAfee Virus Scan:
Right-click the ‘VirusScan’ icon in the system tray, and then click 'Enable' or 'Disable'.
In the 'Command Prompt, type either “Shstat.exe –disable” or “Shstat.exe –enable”.
Click the 'Stop' or 'Start' button on the 'VirusScan Console Toolbar'.
An antivirus program that scans and removes malicious files from your computer, McAfee also scans incoming and outgoing emails as well as warns you of files trying to change your system's configuration. When installing new software on your computer, McAfee On-Access Scanning may block the software's installation process. The scanning utility may also block the installation process from accessing your computer's Internet connection. You can disable the scanning utility and restart it once you are done installing the new software. The entire procedure is applicable to McAfee AntiVirus Utilities.