Itunes?upload videos to Ipod Nano?

I want to upload some videos to my Ipod, but it says they cannot be played on this Ipod. Can I convert them? They're formats are; DCF File, Movie Clip (thats all it says and opens with windows media player), and Video clip. What format do I need to play them on my Ipod? Converter must be quick to download because i do not have broadband and preferably free. The converter in Itunes does not work. I have an IPOD NANO.