if you havn't set a password for an account called 'administrator' on your computer,
when your computer boots up, press F8 repeatedly until you get a screen with startup options. Select 'Safe Mode', but if it comes up with a choice asking you what disk to use, then select you hard disk and press F8 again repeatedly.
After booting into safe mode, click 'Administrator' or if you get a dialog box with username and password, type in 'administrator' (make sure you spell it correctly) and click OK. Now go to the start menu, control panel, user accounts, click your user account and then your account. Now try changing your password. You shouldn't need to enter your old password.