This
 hack will show you how to reset Windows administrator password (for Win
 2000, XP, Vista and Win 7) at times when you forget it or when you want
 to gain access to a computer for which you do not know the password.
Most
 of us have experienced a situation where in we need to gain access to a
 computer which is password protected or at times we may forget the 
administrator password without which it becomes impossible to login to 
the computer. So, if you are somewhat in a same kind of situation, here 
is an excellent hack using which you can reset the password or make the 
password empty (remove the password) so that you can gain administrator 
access to the computer.
You can do this with a small tool called  Offline NT Password & Registry Editor.
 This utility works offline, that means you need to shut down your 
computer and boot off your using a floppy disk, CD or USB device (such 
as pen drive). The tool has the following features:
- You do not need to know the old password to set a new one.
 - This tool can detect and unlock locked or disabled out user accounts!
 - There is also a registry editor and other registry utilities that works under linux/unix, and can be used for other things than password editing.
 
How it works?
Most Windows operating systems stores the login passwords and other encrypted passwords in a file called sam (Security Accounts Manager). This file can be usually found in \windows\system32\config.
 This file is a part of Windows registry and remains inaccessible as 
long as the OS is active. Hence, it is necessary that you boot off your 
computer and access this sam file via the boot menu. 
This tool intelligently gains access to this file and will reset/remove 
the password associated with administrator or any other account.
The download link for both CD and floppy drives along with the complete instructions is given below:
It
 is recommended that you download the CD version of the tool since the 
floppy drive is outdated or doesn’t exist in today’s computer. After the
 download, you’ll get a bootable image which you need to burn it onto a 
blank CD. Now boot your computer from this CD and follow the screen 
instructions to reset the password.
Another simple way to reset non-administrator account passwords:
Here
 is another simple way through which you can reset the password of any 
non-administrator accounts. The only requirement for this is that you 
need to have administrator privileges. Here is a step-by-step 
instruction to accomplish this task:
- Open the command prompt (Start->Run->type cmd->Enter)
 - Now type net user and hit Enter
 - Now the system will show you a list of user accounts on the computer. Say for example, you need to reset the password of the account by name John, then do as follows:
 - Type net user John * and hit Enter. Now, the system will ask you to enter the new password for the account. That’s it. Now you’ve successfully reset the password for John without knowing his old password.
 
So,
 in this way you can reset the password of any Windows account at times 
when you forget it so that you need not re-install your OS for any 
reason. I hope this helps.