Before using John, learn how to use chntpw tool, he comes with BackBox. But to get SAM file you just need to boot with BackBox Live CD, and mount windows partition, then you will find that partition inside /media folder in BackBox. Then go to /media/YOUR WINDWOS HARD DISK/WINDOWS/System32/config/, and you will see file SAM, open it and do what you need to do
. chntpw tool can help you to get all information or remove users, edit passowrd etc ..it's a good tool.