|It would appear that at some time you've watched a VCD (Video CD) in Media Player, and associated the .dat file extension with that program. VCD's use the .dat extension on mpeg1 encoded video files. The same extension is used for data files that are NOT video files. You should change the file association to "none".|
A tip for future use:
When you wish to view video on a VCD, copy the .dat file(s) to your hard drive, then change the file extension to .mpg. The video will then play in Media Player by double-clicking.
The same applies to .vob files on a DVD.
Please let us know if you found someone's advice to be helpful.