I have no idea what the problem with winff might be (I don't even know what winff is anyway). I'd check the file size if you are working in a fat32 file system, since fat will not allow you file sizes over 4gb (-1 byte). Besides that, I suggest using an open source alternative and encoding in an open source media format instead of wma. Mencoder, ffmpeg, avidemux and a plethora of options (both graphical and command-line-based) are available. I'd choose ogg or flac, depending of you want lossy or lossless compression.
I have no idea if the mentioned tools support closed formats like wma, though.