You will need a directshow filter or XviD codec to playback the file.
FFDShow is a filter that should work:
Try this version -- http://prdownloads.sourceforge.net/ffdshow/ffdshow-20041012.exe?download
You can find more up to date versions too.
Also to find what a file is made of you can use one of the tools found in this thread:
http://www.moviecodec.com/topics/5041p1.html
Reading the file name can be helpful too -- hdtv.xvid-vfua says the file is from a HDTV or High Definition source and encoded/made using the XviD codec. VFUA is the the group who made it. 
hello!
Please can you help me? I know this may sound a bit dumb, but I have some files that end with hdtv.xvid-vfua but I have nothing to play them on. I do have windows media player 10, realplayer divx and quicktime but none play it.
Can anyone please suggest a player or some codecs I can get to make it work.
Thanks in advance.
Any help is greatly appreciated
Melvik