Try the WinHelp API call.
 

something like

   WinHelp(Handle,lpstrNameOfHelpFile,HELP_CONTEXT,ContextNumber);

To use help contexts, you need to have a [MAP] section in your Help project
file that defines specifies a reference number for all the topics you would
like to access. The relevant topic number is passed as the ContextNumber to
the WinHelp API call.

[MAP]
New           100
Open          110
Close         120

   WinHelp(Handle,'c:\mypath\myHelp.hlp',HELP_CONTEXT,100);

should show the "Open" topic.