The site
http://vixy.net/ is still in it's BETA version, so I'm only guessing, but the program that is converting the files might still be being worked on.
There aren't very many programs out there (that I know of) that can successfully convert *.flv files to *.avi.
So, if your only getting this problem from this one site, it's probably still being worked on, you can contact the web site and tell them about your problem too, it might be something to do with the converter/settings there using as well.
*I've tried to convert a file into an *.avi before and I got the same problem as you, but that was long ago, that's why I'm suggestion it might be the web site or something.*
Also: Use a program called 'gspot' (
http://www.headbands.com/gspot/) to see if your computer has the correct codex's to play the file, if you do, no idea*
[There is also 'http://media-convert.com/convert/' it converts files for free too.]
Sorry I'm not very helpfull, I tried to help :oops: