windows failed to start

After coming home from class, my PC's programs were all not responding, so I decided to shut off my PC and give it a reboot. After doing this, I got the error "Windows has failed to start" as well as the error code "0xc000000f" and it mentioned the file "Boot/BCD." I have tried multiple things...

Alright, so this began recently. I was doing some home work nothing unusual, then out of nowhere, bluescreen. I waited patiently for my PC to reboot, but when it did a screen saying "Windows failed to start" and an error 0xc00000f. I looked up for a solution and found something saying restart...

i have just updated a game (for honor) but i cant launch it. it tells me there is too little disk space. problem is it is installed on my E: drive, but it is only my ssd C: that is full. im not sure what to do here.

Im building a pc, upgrading a 6 year old build with a i7-2700k i believe maybe it was 2600k with a beastly powerhouse gtx 560 lolz. Anyway I really wanted to get the newest greatest cpu but it's impossible to find and im not paying 200 over list on scambay. I also dont play insanely demanding...

I would just like to know how I would be able to uninstall Windows 8.1 from my PC, as I will be installing a different version of it. Thanks!