Sounds like just that -- no filters to do it.
Windows, especially programs of that era, have various codecs, demuxers and whatever to handle things.
Many modern video programs will use newer standards than the meteo converter will handle, or indeed not install them and have internal filters/decoders (see stuff like VLC).
Fixes
You will probably then want to go find some old school VFW codecs for the input file you are using. I always used to like
https://sourceforge.net/projects/ffdshow-tryout/ but I have not kept up with the ins and outs of windows codecs for a while so there might be something better.
Alternatively. Meteo was unpleasant to work with, even by old school VFW era codec standards, so I quite often first rendered it as raw frames and audio and then converted that with meteo -- as you will struggle to do more than about 5 minutes of real footage or maybe 12 of certain cartoons then it should not be so bad to go raw for this. I quite like virtualdub for this (it has nice options in the video and audio dropdowns to send raw frames/unprocessed footage and audio, and save it as a nice AVI that meteo will handle.
Others have done OK if they first convert to an older standard like MPG1.