Every so often I open a CHM and I get the table of contents but no actual contents. I have looked before for the answer, found it, remembered it for a few weeks – then forgotten all about it again.
Maybe if I actually write it down, I won’t forget again (I learn best by writing). The problem is Vista tracks if the file came from another computer (which is a good thing for it to do), marks it as such and “blocks it”. If you go into properties you can unblock it.