My computer turns itself off?

Hi, i was having some problems with my computer not so long ago where it wouldnt start. After abit of fiddling and constant trying it began to shut itself down on its own when starting up. I put the PC onto its side so it was no longer upright and it worked, I reinstalled windows and then everything was working.

After a month or so i hadnt been able to get my sound device working, when i eventually did get my sound device working my pc is now shorting out again and shutting itself down and wont even come on properly.

Any suggestions?