Well here’s one of cool tricks which I did in my university days.

Method 1

Boot up with DOS and delete the sam.exe and sam.log files from Windows\system32\config in your hard drive. Now when you boot up in NT the password on your built-in administrator account which will be blank (i.e No password). This solution works only if your hard drive is FAT kind.
Method 2

Step1.Put your hard disk of your computer in any other pc .

Step 2.Boot that computer and use your hard disk as a secondary hard disk (Don’t boot as primary hard disk ).

Step 3.Then open that drive in which the victim’s window(or your window) is installed.

Step 4.Go to locationwindows ->system32->con fig

Step 5.And delete SAM.exe and SAM.log

Step 6.Now remove hard disk andput in your computer.

Step7.Boot your computer