Can't boot Windows 8

[JustATest]

Its okay, I managed to disable it in Regedit. But if I simply add my graphics card again should it work?

That's tough to say. It could be a hardware issue or still even a software issue. Maybe it could also be a heat-related or dirt issue. Is you PC dirty inside? If so, try cleaning it with a generally accepted method and see what happens. Good luck.

I removed all the drivers, shutdown my PC and connected up my graphics card but it doesn't show anything, whenever I connect up the graphics card and boot the computer I just get a black screen. It doesn't even show to boot screen. Nothing at all

Did you change the BIOS setting back to PCI-E (or whatever the opposite of onboard is). You did confirm that you moved the moniotor cable to the PCI-E card as well; yes? If you did all that and it still doesn't work, I think you'll need a new PCI-E graphics card. Good luck.