Problem with converting videos for my zune?

I have just bought a Zune (2nd generation) and I have a bunch of AVI video files that I want to convert onto my zune. I used a piece of software called "free video converter" which I have used I the past and it works likes a charm. I read in the instructions that the Zune accepts WMV as its main video file format, so I converted the AVIs to that. I brought them into the zune software and they played perfectly fine in the program, but when I tried to place them into my zune, it just said converting (file name) and then eventually stopped and gave me an error message saying the file is corrupt or invalid.
Can someone help me!