When trying a software called Memories on TV (photo and video slideshows), I was able to create AVI files that play in Windows Media Player and are read by YouTube, but that same Memories on TV is unable to read them.  I emailed support and they said:

It's likely that you do not have a MemoriesOnTV compatible decoder (read)
for XviD, although you have a compatible encoder (save).
 
Try installing FFDSHOW and see if it helps:
 
http://www.free-codecs.com/download_soft.php?d=4787&s=50
 
FFDSHOW is a collection of many decoders which are compatible with
MemoriesOnTV.

I went to that site and saw the explanation for FFDShow:

First of all, the FFDSHOW consumes way less CPU power than original DivX codecs, even with maximum post-processing. At the same time the post-processing seems to give excellent quality results and allows more configuring than the original DivX 5 codec. It can even automaticly reduce post-processing if your CPU is getting overloaded. The noise filter adds a kind of a rasterising effect to the picture - might be useful in some cases. The alpha version also includes experimental playback for MPEG-1 and MPEG-2 files (such as VCD and SVCD movies), which works pretty * well. Only problem is that MPEG-2 playback can't play the SVCD clips directly from the CD, but requires the video to be extracted to the HDD by using VCDGear first.

I am totally ignorant about these things, so I would appreciate any help.  Should I install this package, or could it be detrimental to something I already have?  Does anyone use it?

Thanks MikLav.  So no issues at all? I have the latest DivX installed, downloaded if I am right from Microsoft.  I was surprised to see a software that requires something different. Will this FFDShow overwrite anything?