I have used hirens cd before to achieve this. Hirens incorporates both wingate and thew offline nt password tool. The easiest way is through wingate as it allows you to logon with any password and change the password through the administration tools area natively. However, I have not tried this on windows 8 as yet but it definately works on all nt win2000, Xp, and win7. Just have to rememebr to remove it once finished. Secondly is the nt offline edit, which also works very well. However, you need to be very careful and if it is proper efi boot there is a chance that it will not work. Let us know how you go...I am sure we will be able to fix it. I know of a tool which can also "read" the passwords so this might be another option.
Cheers