Whenever a CHM file is not viewable, I usually can go Properties of that file, click Unblock and then view the file. But this method suddenly doesn't work for me today because there is no Unblock button available.
I found that all the CHM files under that particular directory are not viewable. They were all working fine in the past and I also remember that I unblocked all those files. I am not sure if it is related to my most recent security update last week. The interesting is that, as soon as I copied those CHM files outside that directory, they become all viewable again. It is weird. Those CHM files are not in the network but my local drive. Well, the good thing is that the problem is solved.
In normal situation, see KB902225 to resolve CHM un-viewable issue.