You may find that on occasion you can not view the content of CHM files on Windows Vista. You can see the table of contents but the actual details are not displayed.

This can occur if the file has come from another computer, and the OS now blocks these from running. CHM is not the only file that can be affected I have seen this issue with Javascript files that are used in Vista Gadgets.

You can click the Unblock button and it will allow the single file to be trusted.

In addition, check that the path to the .CHM file doesn't have any # characters in it anywhere. Otherwise, the help viewer gets confused (# means something specific in HTML). If it does, you'll have to move the file somewhere else.
http://blog.crowe.co.nz/archive/2007/04/13/719.aspx