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Which purpose does the file hh.dat serve?

The hh.dat file stores user-specific information on each the HTMLHelp files (.CHM) on your system (position, favourite topics, search history, etc.) including:

  • The most recent size and position of each help window.
  • The most recently used tab in the navigation pane.
  • Any topics that the user has added to the Favorites tab.
  • Any search terms that the user previously submitted on the Search tab.

This hh.dat file can cause a error if it has somehow been corrupted. Delete or rename the file hh.dat to reset all (!) CHM windows on your system to their default settings.

You should find hh.dat in this hidden directory:

Windows XP: 

\Documents and Settings\%username%\Application Data\Microsoft\HTML Help
Windows10: 
\Users\%username%\AppData\Roaming\Microsoft\HTML Help
Windows will create a new version of hh.dat when you next open any .chm file.

The unofficial CHM spec seems to indicate that hh.dat is an archive of the same type (ITSF) as CHM itself.