Boot your computer using a DOS or Windows 9x boot disk.
Type:
CD C:\winnt\system32\config
and press Enter to change to the folder where your passwords are stored. (Of course, if you've installed Windows to a folder other than C:\winnt, you should substitute the correct folder name in the command above.)
Delete the SAM file by typing:
DEL sam.*
and pressing Enter.
Remove the boot floppy and restart your machine.
When prompted for a logon name, use Administrator with a blank password.
The SAM file defines all the user accounts on the computer. When you delete it, you'll lose all user accounts.