How to change computer password without knowing the old one

There are some ways but i will tell to:

1.open command prompt(it is in accessories)

type:

-net user

-this command will show you the users of the computer
-Then type :net user (the name of the user you want to change the password) *
-then type the new password and then reconfirm it.

2.in my computer icon right click

click manage

in computer management go to local users and groups and open users (at the right of the dialog box)
than users of the computer will be shown up.Right click in the user that you want to change the password and then click set password.Type the password that you want.
 

Warning: It only works on Xp,Vista,Windows 7