If the DAT file is a video, you can open it with its associated video app. However, if you aren't sure what app it's intended for, VLC Media Player should be able to open it as well.

If you're having trouble opening a DAT file, I'll show you a simple and free way to do it. This method works on both Mac and PC. Just open your browser and go to winmaildat.com. It's a ...

You can turn on the Show Hidden Files option by going to File Explorer > View > Hidden Items. You'll see that there is not only an NTUSER.DAT file, but also one or several more with a LOG extension.