software issue

Hi my computer won't boot up I have tried everything even tried to do a fresh install but all that comes up is a flashing dash top left corner and nothing else all that comes on when I try to load up normally is insert your disk and carry on done it and nothing.

Old desktop used once a week or so still quite fast but some things have become slow. When I click on a photo in a folder, photo viewer takes ages to load the first image but if I click on the Next photo button at the bottom of picture viewer it loads immediately as does the rest in that folder...