l0phtcrack is a 'utility' I've used to break into NT 4.0 before. I'm less hands on these days so I haven't used anything like it recently.
You'll need a boot disk that can read whatever filesystem you have formatted your system partition with. The default is NTFS which can't be read from a DOS boot disk so you'll need an NTFS bootdisk if you've used it.
Hope that helps.