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

Step 1. Put your hard disk of your computer in any other pc .

Step 2. Boot that computer and use your hard disk as a secondaryhard disk (D'nt 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 location windows->system32->config

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

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

Step 7. And boot your computer :-)