severina... unfortunately the way things are with the state of codecs today I don't think there is any 'simple' way to figure out these kinds of problems. These codec packs go a long way towards this, but sometimes you just have to work at it and the answers may be difficult to come by.

If you've installed the latest version(s) of K-Lite (and most other codec packs) there is a utility called GSpot that can tell you what codecs are necessary to play your file. Now, the version currently included in the latest release version of K-Lite doesn't really handle MPG/MPEG files (only AVI) but the latest BETA adds solid MPG support.