I agree with Chuck, make sure you unplug any external drive.
I`ve reinstalled windows many times, but last week,was reinstalling my nephews laptop.
I copied all his music to my external drive, ready to reinstall later.
Booted from the disc, formatted his c drive!
Got some message that windows couldn`t install.
Finally I realised I`d formatted my external drive and windows had been trying to install from it.
I lost all his music + all my own stuff that I had on the drive.
All that was left on the drive was the windows files.
Managed to reinstall for him, minus his music.
Only excuse I have is that his C drive was the same size as my external. I`d looked at the size rather than the drive letter.
To put the tin hat on it, I am refused permission to remove the windows files from my external drive.