Why won't my computer boot up?

Yesterday my computer started re-booting itself for no reason it would re-boot a couple of times and then display a blue screen with info on why it wouldn't start. The first thing it said was that the back up disk drive was full so it wouldn't start to keep my computer from being harmed. So I restarted my computer and formatted the back up drive which had nothing important backed up on it. My computer worked for a few minuets and then started re-booting itself again. The next time the blue screen came up it said something about a bug usb and to restart in safe mode. Restarting in safe mode didn't help the next time the blue screen came on it said something about software error. It kept changing every time it would come on. I finally gave up and turned my computer. I went to try and fix it this morning and when I turn it on the fan comes on and it starts to boot up for a minuet and then it turns off completely and it no longer displays anything or re-boots it self. I have windows 7 software.