Can a computer freeze because of the peripherals or is there something wrong with the hard drive?

A man who fixes computers says the screen keeps freezing due to a fault in one of the peripherals (monitor/printer/keyboard etc.) but he can't seem to pinpoint the problem or fix it. I only thought that this happened if there was a problem with the hard drive (e.g. virus). What are the main reasons for a computer freezing? Is this man a waste of time?