Cracking the password hash is very difficult, but replacing the password hash is almost instant. This is why PCUnlocker can reset lost Windows password in no time, while Ophcrack will take a very long time to recover a password.

Cracking is important in networks, so you can access all the computers programatically

Yes, but it is suspect if the user wants to login again, its password has changed..

But you can always save the hash and put it back once you're done.

Sorry but how?