Laptop behaving abnormally

My acer laptop is behaving abnormally. It runs normally for some time, later everything goes blue displaying message ' windows has shut down abruptly to prevent permanent damage to your computer .A process or thread crucial for system operation has terminated. ..............'

When i tried restarting system , this time system hung up and command prompt automatically opened displaying a rotating'/'. When i pressed esc it displayed 'system reboot has been terminated ' and later displayed message 'no os found'.

I scanned drive for errors and could nt find any.

Also system is turning on dvd driver of lappy is becoming active as if it contained a disk.

Someone please tell me wats happening,....