My .dat files have somehow become associated with Nero. How can I remove the
association and have the files with no associated program as it was originally.
Actually, .DAT files can be anything (video, data file or something else.) By default, .dat files are not associated with any application. If you've accidentally associated .DAT files with Nero or Windows Media Player, you can reset the associations to defaults.
Click Start, Run and type this:
reg delete HKCR\.DAT /ve /f