cptkirkh:
ntuser.dat is part of profile file, most likely (from how it sounds) of a currently logged on user (administrator, in case of server). If you REALLY want to back it up ( and I wouldn't understand, why,since it's not of any particular value, unless your administrative account has an extremely complicated configuration), you can purchase a special backup software, that is able to backup SAM database (I would recommend Veritas), otherwise, you can to try to:
1.schedule to backup this only ntuser.dat file in about, say, 3 minutes;
2.log out of the box, and wait for 5 minutes;
3.log back on, and check out the log file.
ntuser.dat is always open when someone is logged into the station (each ntuser.dat for each user), so it cannot be backed up ,since it's open all the time.