well that depens on your version of windows... in Windows 1, well, there was no password in the first Windows 😉
In 95/98/Me they are stored in \windows in a .pwd file for each user name (delete that file and you have to reenter a new password)
In NT/2000/NET they are encrypted in a .SAM file, and you have to go in from outside of windows to do anything with that.
For more info, search the archives at neworder.box.sk and welcome to the world of password hacking
🆒