VirtualDub will open XVID, but you have to trick it into doing so. XVID is a competing codec with Divx (notice it is Divx spelled backwards).
Each avi file has what is called the "fourcc" code that indicates what codec should be used. There is a little freeware utility called Avic that will allow you to change it.
You can find Avic at
www.inmatrix.com