We inject scripting into the actual help file which is designed to point back to the html file and execute it. Provided the html file name contains the 'word' chm in it, Internet Explorer 5.5 and 6.0 executes it.
Because it is an html file proper, Internet Explorer opens it. The scripting inside is then parsed and fired. That scripting is pointing back to the same file with a showHelp call and because that is precisely what it is, it executes !
Tested on win98 IE5.5 and IE6. Constructed for win98:
[harmless *.exe - requires positioning of Mshta.exe @ C:\WINDOWS\SYSTEM]
Be aware of html files out there.