Changing a user's password without knowing the old password

Press 'Start'.
Press 'Run'.
Type 'cmd'.
Then type 'net user'.
A list of users in your computer appears.

Type 'net user accountname *'.
(accountname is the name of the user account, one of those displayed in the above steps)

Then type the new password and press Enter.
You will be asked to retype the password for confirmation.
Type the same password and press Enter.