Here's an ancient .HLP file decompiler that can decompile reliably old help files
of any size into a .HPJ project. .HPJ projects are also outdated but they are easily converted by MS HTML Help Workshop into newer .HHP projects compilable with the Help Workshop into modern .CHM files.
Please scroll to the bottom of HELPDECO.C to see the command line options that HELPDECO.EXE supports. Be sure to have the .CNT content file decompiled for the .CHM help to have a usable Contents pane. Note that it will require a
separate run of HELPDECO.EXE with the
/c command line option.
SPLITMRB.EXE is claimed to be able to split old .MRB multi-resolution bitmaps into separate BMP's but this app fails too often to actually do it for you. You'll be much better off using your MS Paint to copy-paste such bitmaps directly off screen.
Note also well that HELPDECO.EXE is incapable of coping with a .HLP file's popups and/or non-scrolling areas should they appear in the .HLP file that you're trying to convert.
The only freeware program that claims being able to restore the popup functionality (no non-scrolling areas though) is
HHPMod that you can google for in the net.
Be forewarned however that it is painstaikingly slow (on the order of hours for multi-megabyte files) to do its job and you can
never know if it is going to succeed or not. Do not believe the over-detailed and somewhat narcissistic manual that comes with it -- the proggy just doesn't do what the author claims.
The only other application that claims to resolve both non-scrolling area and popup issues is Adobe RoboHelp that will come to you eagerly in exchange for a few hundred bucks.
Happy conversion!
.