If your version of AVG is a real-time, active anti-virus scan, I would definitely turn it OFF, when editing. Some of those programs (do not know AVG) will lock any file being read from, or written to, and with video editing, lots of files are being created, and read from. Real-time scanning by any "anti" program can really mess up a video-editing session.
Good luck, hope that is the culprit, and sorry about the lost 4 hours of editing,
hmmm thats a catch22 situation - viruses are an issue.... also the system is on a network ... but why this time and why now... since you are an expert have you ever heard of any ever encoutering such an issue.
Actually, I am anything BUT an expert.
I agree that virus infection can be a problem, however I set up a hardware firewall for my entire network. Nothing gets through, unless I invite it through. Hence, when editing, with all of my "anti-anything" programs shut down, I am not surfing the Internet, nor am I doing any e-mails. I am only doing editing.
With real-time scanning, many of the programs will monitor all reads and writes, holding up the processes of the NLE program, while every read and every write is studied. Also, access to many files can be blocked, until all of the surveying is complete, and with large video files, that takes time.
I also turn OFF anything, that I am not using, while editing.
As a test, I would disconnect from your Internet connection, turn off your anit-anything programs and try the same operations.