Автор: http://www.swissdelphicenter.ch

function NetUserChangePassword(Domain: PWideChar; UserName: PWideChar; OldPassword: PWideChar;
   NewPassword: PWideChar): Longint; stdcall; external 'netapi32.dll'
   Name 'NetUserChangePassword';

 // Changes a user's password for a specified network server or domain. 
// Requirements:  Windows NT/2000/XP 
// Windows 95/98/Me: You can use the PwdChangePassword function to change a user's 
// Windows logon password on these platforms 

procedure TForm1.Button1Click(Sender: TObject);
 begin
   NetUserChangePassword(PWideChar(WideString('\\COMPUTER')),
     PWideChar(WideString('username')),
     PWideChar(WideString('oldpass')),
     PWideChar(WideString('newpass')));
 end;