Every user profile created on a Windows operating system has an Ntuser.dat file. A user profile contains personal files and preference settings that are specific to each user. For example, each user profile contains a unique Documents folder, custom settings, desktop properties and browsing history.
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The Ntuser.dat file is a registry file. Each user's Ntuser.dat file contains the registry settings for their individual account. The Windows registry, as described by Microsoft, is a "central hierarchical database" that contains information about the software, hardware and user profiles contained on a computer. Windows is constantly referencing its registry files throughout the course of its operation. The configuration of the "HKEY_CURRENT_USER" branch of the registry is supported by the current user's Ntuser.dat file.
The Ntuser.dat file is in your user folder. In Windows 8, Windows 7 and Windows Vista, user folders are at C:\Users\USER_NAME.
The Ntuser.dat file is a hidden file. Hidden files, as the name implies, cannot be seen unless you configure your folder options to show hidden files. Therefore, if you do not see your Ntuser.dat file when you initially open your user folder, press "Windows-E" to open Windows Explorer and then click "View" on the ribbon bar. Click "Options," and then choose "Change Folder and Search Options." Click the "View" tab in the Folder Options window, select "Show Hidden Files, Folders and Drives" in the Advanced Settings pane and then click "OK."
Deleting a Ntuser.dat file will delete a users preference settings and may corrupt the user's profile. Because the Ntuser.dat file is a system file, it is normally locked to protect it from accidental deletion. Only a system administrator operating logged into an administrator-level user account that is different from the account that contains the Ntuser.dat file you want to delete can delete a Ntuser.dat file.
The Ntuser.dat.log file, which is listed as a text file, backs up the Ntuser.dat file. If an error occurs in the Ntuser.dat file, Windows uses the Ntuser.dat.log file to correct it. If a computer is part of a network, it may also have a Ntuser.ini file. Ntuser.ini files are used to create a roaming profile for a user that does not get copied to the network server. When the DAT extension of an Ntuser.dat file is changed to Ntuser.man, it creates a mandatory user profile that is no longer user-configurable. In this case, whenever a user logs off any changes made to the user's preference settings are not saved.