I may be wrong, but won't the K-Lite pack work? Most of the codecs there should work on virtually any Windoze machine, I would think. The other possiblility is to get the DivX and even XdiV codecs themselves, since most so called AVI's are one or the other. Another possiblility is to convert the videos to another format, such as mpg or flv. flv's, by the way, will play on Media Player Classic, which comes as part of K-Lite. MPG's are big, but they should play on anything including the kitchen sink!
You could also try getting the VLC Player. It comes with its own codecs. I know I used to use it on Win98. It should be available on any good software site such as Snapfiles, Tucows, etc. About the only thing that it won't play are wmv and the Flash series. For the latter, you can find a swf-flv player on the above sites.