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I recently could not log in to a pc when its trust relationship with the domain was broken and I did not have the local username or password. A colleague recommend a utility called NTPassword which can be booted from a CD, USB, or floppy disk. With this I was able to clear the password of the local administrator and also enable the account. It is linux based and doesn’t look very pretty but it is menu driven and relatively easy to use.
Paul Hartigan's Blog 