writing help pages

How are the help pages provided with applications usually written and linked into the application? I know I should probably write them in HTML but then how do I make them accessible from a C application via the help menu?
If you don't want to bother explaining here I would appreciate pointers to any resources or examples online - I couldn't find any.

thanks

The application works on both windows and linux.

you can get the windows help file author and compiler which allows you to compile proper windows help files with search, contents etc. However if you want it to run cross-platform your'e probably better off using html pages.