There was a thread on this just a couple of days ago. Here's the solution posted by John Thornton:

Buried very deep in the HTML Help Workshop help file is the following way to call the help program and specifiy the tab you wish...

start your search with "API reference, HtmlHelp"

HH_DISPLAY_INDEX command
Selects the Index tab in the Navigation pane of the HTML Help Viewer and searches for the keyword specified in the dwData parameter.

Example API Call
HtmlHelp(
         hwnd,
         "cat.chm",
         HH_DISPLAY_INDEX,
         (DWORD)"meow") ;

Return value
The handle (hwnd) of the help window.

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>Okee, I am trying to hook a compiled HTML file to my applicatoin. On my menu, I know you can specify bars for Contents, Index, ans Search tabs of the compiled HTML file. I am wondering if I need to set the defines up somewheres in the compiled HTML project to allow me to choose these tabs. What I'm trying to do is when the user clicks on "Contents" in my menu in the application, the "Contents" tab is the one that pops up. What happens now when I click on the "Contents" in my menu, the compiled html displays a blank grey box on the left. When I click on "Search" on my menu, the contents shows up??? What the heck? The "Search" on the menu is tied to _mst_hpsch and the "Contents" is tied to _mst_help. Are the _mst_??? documented somewheres? Any help (non-psychological) would be appreciated. Thanks.
>
>George