After downloading Norton 360 and version 4 my computer now refuses to turn on

After downloading Norton 360 and version 4 my computer now refuses to turn on.  After a successful download Norton asked me to restart my computer - I clicked restart and so I did. Now my computer refuses to start.  I get the initial screen that gives me the option to press F2 or F12 - neither of which I have any idea about.  Then after that page has loaded the screen goes blank and the computer just keeps whirring away and showing nothing.  I have tried pressing F8 but the safe mode page does not show and I can press nothing.  The only thing that is shown is a flashing line in the top left corner.  If I continually press ENTER/RETURN after pressing F8 then I for a split second see the safe mode page - but then after a minute or so the computer switches off.

 

PLEASE PLEASE PLEASE HELP - HAS ANYBODY ELSE HEARD OF THIS!!!??????