Windows doesn't have a default password that you can use when something goes bad. You will have to remember it or connect to the Administrator account. Reboot the PC and then press F8 until the service menu appears. Select Safe Mode and let it boot. Choose Administrator and wait to see if you need a password or not. In case you require a password, you need another computer and OphCrack LiveCD to get the password for the default users.
Download OphCrack Live CD: http://ophcrack.sourceforge.net/