[BSPeter]

You're reviving a 7½ year old thread!! That's what spammers also do!!

The subject of this thread concerns (divx).avi movies, which, however, have an incorrect .dat extension (for whatever reason).
These files (only having an incorrect extension) can be played in BS.Player without renaming, by opening them from BS.Player's control panel.
Like:
Click on [Open file]-button.
At bottom change Filetypes from 'Mediafiles' into 'All files'.
Search and double-click the .dat file (or select and click on [Open/Run]-button).
That's it!

Now, you on the other hand want to convert a dat-file to avi. Needless to say that this isn't, in fact, a matter for BS.Player forums.
However, you also do not mention what the actual contents of the .data-file are. So how is anyone to know what to convert from?
(Simply renaming doesn't work? You're sure its media? Perhaps zipped? P.S.: What's wwm?)