Try this, open your Explorer (Windows E);
Get to the Folder options (I think that would be under Tools, or if not Tools, try View);
In the Folder Options, select the "File Types" tab;
Scroll through the Registered file types, and perhaps you will find in the alphabetized list DAT,
if not then the hard way is to select the first one and look at the "Opens with" line,
if it says "Winword" or probably "Microsoft Word" then see if the file type associated with it is DAT.
(to progress through each of the types - you only need to press the Down-arrow key);
If it is associated with DAT, and nothing else, just delete the type. Then you'll be back to the way you were.
If it is associated with DAT and also some other file types then see if you can Change or Modify or Edit it, to remove just that DAT association.
I was trying to open a .dat file and forgot to uncheck always open with.
Now my dat files have winword icon instead of a windows flag.
I tried holding down shift and right clicking open with, however the windows icon flag is not there.
Any suggestions appreciated.