It's a slim one but my next move given only the clues so far is to try this in SAFE MODE. If that fails I'm off to run system scans if uninstalling the security suite doesn't bring it back to life.
This is the first solid clue that something is borked with this OS on this PC. Up till this point it sounded as if it's the old issue and discussion that we have to slog through.
Any more clues would be needed for me to not think this is some infection or antivirus protection.