I'm gonna put forward a PSU problem ... have you tried a different PSU to see if that would make a difference? Usually if you're having problems all over the place it's either your motherboard or your PSU ... I highly doubt that it's your video card if you're having problems with Windows, for example. I'd rule out RAM as you say you're having issues in the BIOS/startup screens ... a simple reflash of the BIOS might be able to fix/alleviate some of the problems (slightly corrupted BIOS, for example). If that doesn't work, then I'd say yes it's either your PSU or your motherboard.