I recently had to fix a changed Administrator password on a Windows Vista (most probably by malware, it disabled all admin users and changed passwords).
One can only use the Ubuntu Desktop installer with chntpw, but it is large and it's pointless to use it for this task. I found a very simple distro to use for the same task.
There is one tool I would not recommend: Ophcrack. It is supposed to "hack" (actually an unoptimized bruteforcer). I couldn't find a way to just "change" the pass. It didn't have chntpw bundled.
Here's a link: http://pogostick.net/~pnh/ntpasswd/ (it is very small, AFAIR only 3-10mb).