Hello
Unfortunately I had the same problem on a laptop. I downloaded the bootable CD version and used it to boot the machine. The program worked fine until it got to the "sam" file which should contain the usernames and passwords. Unfortunately, the message said "there are no usernames in this file" - this was immediately after a full install when I deliberately put in two usernames. Tried every other "sam" file in the windows directory and all subdirectories and couldn't find any usernames anywhere.
In the end, I used a Knoppix CD (works entirely off the CD) to access the NTFS partition (where all the data files were) and copied all the stuff across a network to my desktop PC. Once the data was safe I tried a few things on the laptop hard-drive. The outcome was that only a complete format of the drive (I actually re-partitioned it) and a full Windows XP re-install from scratch would cure the problem. Good Luck !!
Regards
Millyfish