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.

However, if you'd like to see it out of curiosity, here's how to find it: Go to File Explorer and browse to C:\Users\*YourUserName* If you don't see NTUSER.DAT yet, it's okay. Microsoft has hidden it ...