Computer shutting down

My computer keeps shutting down randomly. It usually happens after about 30 mins or more and the whole comuter shuts down and the hard disc heads are removed (you can tell by the sound).
What is going on?
Is it a virus?
I have scanned my system with antivirus like 4 times, got some virus and they’re in a ‘Virus Chest’, but it still shuts down. What do i do with the things is the ‘Chest’ and is it something else?

**Help me!!!