If you come across a file on your Microsoft Windows computer called ntuser.dat, don't delete it. It contains user-specific configuration information that's read by Windows when you log in to your computer. It's a copy of information that's also stored in the system registry while you use Windows.
The ntuser.dat file stores user profile information used to configure Windows for different users. There's usually an ntuser.dat file stored in the home directory of each Windows user. The name comes from Windows NT, an older Microsoft operating system from which modern versions of Windows evolved, and the tradition of ending data files with a .dat extension. Essentially, the ntuser.dat file contains information about users of all Windows versions since Windows XP through Windows 10.
The data in ntuser.dat is copied back and forth between the file and the Windows registry, a database used by Windows to maintain settings for the operating system and other software on the computer. The registry is divided into sections known in Microsoft terminology as hives, and the ntuser.dat file is a copy of the data stored in the registry hive for a specific user, organized in a set of hives called HKEY_USERS. When you are logged in, your user hive can be found in the registry as HKEY_CURRENT_USER. You can edit and browse the Windows registry with a built-in tool called regedit, but it can be risky to do so if you're not sure what you're doing.
Working With Ntuser.dat
It's also not a good idea to directly alter or delete the ntuser.dat files on your computer. Those actions can cause you to lose information about how Windows is configured for specific users and make it difficult to save new configuration information. If you are a system administrator and want to prevent users from permanently altering their Windows configurations so that configurations reset when people log out, you can rename particular users' ntuser.dat files to ntuser.man. The file will still be read by Windows as is, but user configuration changes won't be saved to it for future use.
You can, in theory, load an ntuser.dat file into the Windows registry to edit it. Use a command line tool called reg to do so and to save your changes to the file afterward, but use caution and document the changes you make because it may be difficult to determine what was changed if something breaks.