ntpasswd
This is a utility to (re)set the password of any user that has a valid (local) account on your Windows system. You do not need to know the old password to set a new one. It works offline, that is, you have to shutdown your computer and boot off a floppydisk or CD or another system. Will detect and offer to unlock locked or disabled out user accounts! There is also a registry editor and other registry utilities that works under linux/unix, and can be used for other things than password editing.
Tested on: NT 3.51, NT 4 (all versions and SPs), Windows 2000 (all versions & SPs), Windows XP (all versions, also SP2 and SP3), Windows Server 2003 (all SPs), Vindows Vista 32 and 64 bit (SP1 also), Windows 7 (all variants). Some say also Windows Server 2008 is OK.