Ipod Nano-Touch videos

Hello everyone!
I'm sure this subject has been discussed a lot, and I hope you guys won't kill me for submitting it another time!
Ok here it is, I just can't figure out how to put videos in both the devices.
I ONCE, just ONCE, managed to put an MP4 video into the ipod nano, but that's it, just that video, the other MP4 videos won't enter the device (same format, so why?).
So my question is: is there a program, or a way to convert/obtain MP4 videos without paying, registering and blabla?
Because I'd really like to watch videos on my ipods, the least I want is to use them at their fullest!

Thank you!