".DAT" is a generic file type that could have been created by a lot of different applications - so there probably isn't a specific program for it.
On my system I have a Notepad shortcut in the Explorer context menu that I use to look at generic files like this for which a specific program isn't available. You can set it up like this:
-> Start -> Type "shell:sendto" in the search box, hit Enter.
-> Right-click inside the Explorer window that opens and select "New -> Shortcut"
-> Type "Notepad.exe" for the location
-> Type "Notepad" for the name
Once you've done this, you can right-click any file in Windows Explorer and select "Send To -> Notepad"
Whether or not Notepad can display the file intelligibly depends entirely on what the file contains.