Hey,
Something all of you experienced is that some CHM Help Files don't seem to work correctly sometimes. You do see the Table of Contents of the help file, but the content window just tells you "Page cannot be displayed".
If you use Network storage, like at work, at school or even at home with the new network storage devices then I know what to tell you, simply don't use it to store your CHM ebooks.
It seems that when a CHM file is opened from a network location then it cannot be read. If you copy it to your local hard drive and try to open it, you will see that it works fine.
While this tip is short and simple it might save you a lot of headache. It can also save you 10 years of your life if you don't have the frustration to figure out why all damn CHM files don't seem to work on your machine 
Hope this helps you out,
Cheers,