Hi there,
This problem is encountered regularly on machine with user profile set. But, you've stated that you are running machine with no logon, which would mean that there are only one USER.DAT, SYSTEM.DAT etc. in your WINDOWS directory.
The system registry is built upon USER.DAT, SYSTEM.DAT etc., which is combined to form the system registry during startup of Windows and which would be splitted-up during shut-down.
During one of these two processes, anything could happen. But in yourcase, I would assume that during shut-down and split-up of the registry to the files SYSTEM.DAT and USER.DAT, your machine had hang and you forcibly reset the machine which caused a DEFECT USER.DAT.
Should you have a backup of the system registryfiles, you could restore this using the utility SCANREG.EXE,
SCANREG /RESTORE
though, you have to start it using the Emergency Repair Disk first before using this utility.
If it help, please let me notice.