Some security update for Windows XP blocks access to chm files. To fix this you can right-click on the CHM file, select Properties, and click on the "Unblock" button. Click "Apply" to show the content.
Additional information available at: http://support.microsoft.com/kb/896054/
Sometimes system file \windows\system32\hhctrl.ocx gets missing, corrupt, or unregistered. To register correct hhctrl.ocx file run the following command: regsvr32 hhctrl.ocx
Check registry settings and clear old user settings.
HKEY_CLASSES_ROOT\.chm key should be "chm.file"
HKEY_CLASSES_ROOT\chm.file\shell\open\command should point to existing hh.exe file.
Delete file hh.dat from \Documents and Settings\%username%\Application Data\Microsoft\HTML Help\
Delete file windows.chw from \windows\Help\
Also make sure you do not open the file from network share or desktop, and that path to file does not contain special characters like '#' or non-english characters.
If nothing helps
If you have tried all of the above but could not solve the problem please inform us about it and provide the following information:
- What is your OS, including service pack and localization
- What is your development environment
- When you click on a chapter in TOC, nothing happens or you get "Page not found" topic, or "Action canceled"?
- Can you open other CHM files on same machine?