Recover Windows XP Product Key from ntuser.dat in Corrupted or Unbootable System My Digital Life
provides a link to a download to read the .dat file
but
the NTUSER dat file is actually one of your Registry files. Unlike the other Registry files, NTUSER.DAT is stored in your personal Documents and Settings folder and contains the entire contents of the HKEY_LOCAL_USER branch of the Registry. You can’t delete it because it’s in use and protected, and you wouldn’t want to because otherwise you’ll mess up your whole computer! It will grow as you install more software that creates keys and sub keys in this branch of the Registry, and so is perfectly normal. To summarise: leave NTUSER.DAT alone.
In Windows XP, 2000, and 2003 there are several Registry files. These files are named without a file extension and are stored in the Windows\System32\Config folder. These files are named Software, System, SAM, Security, Default, and UserDiff. There is one more Registry file and it does have a file extension, NTuser.dat. In Windows XP, 2000 and 2003. NTuser.dat is stored in the users folder under the Documents and Settings folder. Each user has their own NTuser.dat file. The NTuser.dat file stores all settings that each user selects; these settings will override settings stored in the System file.
The function of each file is different. Security stores information about security. The SAM file stores information about the Security Accounts Manager service. Neither of these two files, Security and SAM, are viewable in RegEdit, unless you reset the permissions. System stores all the information about hardware. Software stores information about your software and how Windows will perform and the default Windows settings. The Default file, stores all the default user settings, the NTuser.dat file overrides the default user settings. The UserDiff file stores information about the corresponding SubKeys in the HKEY_USERS Hive for each registered user.