For some reason, McAfee System Guard started getting disabled. In my case, a combination of regular McAfee Security Center update and a run of McAfee Virtual Technician seem to have cured the problem (although Virtual Technician found no problems of any kind). Other people, judging by posts in McAfee forums, are less lucky.

Just in case, here’s another possible fix suggested by McAfee support:

• Click on the Start button, then on the Run button.
• Type Services.msc in the Run menu. The Services console will come up.
• In the list of services, find McAfee SystemGuards and right click on it, then select Properties from the menu that appears.
• Restart the service.
• Click on the General tab, then select Automatic option from the Start up type drop-down box.
• Click on the Recovery tab, then select Restart the service option from all drop-down boxes (First failure, Second failure and Subsequent failure).
• Click on the Log on tab. Check the option Allow services to interact with desktop.
• Click the Apply button, then the OK button.

Also, here’s a summary of possible fixes from McAfee’s forums.