My Nokia C3-00 says video not supported when I run videos in AVI format.
nokia c3-00 does support avi files, but you have to make sure, it is compatible to your cell,
check if a preloaded avi file is there on your cell.
convert your avi file according to that file.
also remember the video resolution of your file should be 320x240, you cant play your 640x360 avi files directly.
use any of the video conveters mentioned above, but avi files can be played, try to convert avi into your customized avi first.
I don't have c3 right now, else I could have told you the exact specification to make it work.
you should use format factory.
If the phone's firmware doesn't support the codec, you will need to convert the video to another format.