The hh.dat file stores user-specific information on all the HTMLHelp files (*.CHM) on your system (position, favourite topics, search history, etc.), and 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 directory:
Documents and Settings\%username%\Application Data\Microsoft\HTML Help
Windows will create a new version of hh.dat when you next open any .chm file.