Microsoft's HTML Help Workshop (the tool for creating .chm files) also allows you to decompress them. One problem you may run into is that .chm files are made up of many (often poorly named) html files, so once you decompress them, it may be tough to get them combined back together.
One thing to do is to write a program to walk the .hhc (table of contents file) and combine the separate files into one large html file. The .hhc is actually an object tag heavy .html file, so it's easy to get through.
Another option I've used in the past is to use a product called
chm2pdf to spit the .chm file out as a .pdf file. I then take the .pdf file and print it to RepliGo and then use RepliGo to read the file on my PocketPC.
bsr