winamp has no native mpeg 1(video) decoders and relies on windows for playback so if your windows decoders arn't working properly then neither will winamp for these file formats.(although sometimes you get different results with different players)
try installing ffdshow codec's, i'm guessing its a decoder problem in windows.(and it covers a lot of missing decoders aswell)
if it doesn't work you could try setting libffmpeg for all mpeg codecs in the installer. or it might work just after installation.
an mpg file is very ambiguous and it covers LOTS of different technology as its just a file extension. different decoders will behave differently in the case of a faulty video files and might crash the software or cause visual errors etc.