Haven't found any freeware that works with Xtube yet (which is why i pretty much stay away from it). But found some freeware by eRightsoft called Super (c) that converts Pornotube flv files to other formats. (The Xtube flv files seem to be missing some component that allows the conversion). Maybe someone else on here knows how to make it work.
When you click on the download helper link in your toolbar on the top of your browser, it will give you two files. One looks like a briefcase that is on the bottom and on top is the other, a weird geometric symbol, click on the lower one that looks like a briefcase and it should work.
When you get to the SUPER (c) program, don't use the ffmpeg encoder, use MEncoder instead. Select MPEG-2 video and keep it in the native resolution (no change). Make sure to use the default audio properties in the AUDIO section as well (44100 Sample rate 2 Channels at a 128kbps data rate) and you should be off and running b.c.!!!!