I'm not sure of the source of the problem - occurs only when I'm browsing the internet.  Pages at some sites hang on opening without an error message. Sometimes when I attempt to reload such a page, Explorer freezes.  I can't shut it down in any way, nor can I shut down the computer.  Often when this happens, I also lose sound.  Twice I have got the dreaded blue screen.  I am using all OEM software from Dell and have not installed anything else.  I'm baffled.