Breaking old windows pass words

I was giving a few old computers and i need to break the passwords for them. Two of them are win xp and one is win vista. Is there some software i can use or an exploit i can use to break them? I was given this and now own them i just need to break the passes so i can get the product keys so i can reinstall windows and have a clean start with them.

Or better yet just recover the keys from the registry.

kk ty.