For one you can't crash someone by using their key, at most just auth fail them. And it's your friends fault anyway for probably giving it out.
The only thing that really screwed up any installation of windows for me is ntldr. If that gets ruined/goes missing, you're screwed.
Related note: After inserting RAM into my old computer I booted and saw NTLDR is missing, reboot ctrl alt del etc. I shat bricks, and I'm like OMFGOMFGOMFG, so I made sure the wires were connected n all, still didn't work. Tried different SATA cable and bingo.