Can someone point me in the right direction to a guide that joins XVID files together, or tell me of a program that does it.
For raw XviD files I guess copy /b would work. I assume that the XvID stream is in some kind of container though. Joining depends on the container not what codec the video stream uses. If it is an avi try AVI-Mux GUI or VirtualDubMod.
Give this progarm a try, Advanced X Video Converter (http://www.aoamedia.com/videoconverter.htm ). Lauch the program > Add Task > Join Video Files > set the output to AVI and select XviD codec from the drop-down list.