How to ensure some frames are in correct aspect ratio?

[ANOther1676]

I taped some programmes off DVB-T, and when playing them in the media player I use (KMPlayer) they come out wrong.
I accidentally set aspect ratio of 16:9 on the DVB-T viewer, but that's fixed now, so future recordings won't have this error.

However, what I want to do is ensure that frames that are meant to be 4:3 come out in that size rather than 16:9 - and what's the best free program to do this?

I'm not sure how to progress with this so any advice is welcomed!
(btw; file is in MPEG-2 format saved as .mpg extension)

[jagabo]

I'm not an expert on DVB-T, but I thought the "ripped" MPG files included the original aspect ratio flag. Any player can examine that flag and display the video correctly. Make sure KMPlayer is set to respect the AR flag. Right click on the KMPlayer window and select Screen Controls -> Keep Display Aspect Ratio.