Windows 10 does not support .hlp Help files. Some older programs are still valuable, even if they are no longer supported by the developer, but if they rely on WinHlp32 then their Help files will not be accessible under Windows 10.
Fortunately, there is a work-around. In 2009, Komeil Bahmanpour wrote an excellent command file to install WinHlp32 under Windows 7 well before Microsoft finally provided an official download. Although there is no version for Win10, the Win7 version will also work on Win 10 with a small change in the CMD file and the correct WinHlp32.exe version (it seems that not all versions of WinHlp will work).
So using Komeil's approach, WinHlp32 can be used with Windows 10 with only a minor modification to his original CAB file. As a convenience, this zipfile contains Komeil's original command file which has been modified for Win10, along with a copy of a working version of WinHlp32.exe. To install under Win10, simply download the zipfile and unzip it, then right click on the command file and run it as an administrator (for the necessary rights). Quick, simple and it works. Thanks to Komeil Bahmanpour and those contributing to his blog for this handy tool.
There are always potential security concerns that one should consider and this has been described as one reason why Microsoft changed from hlp to chm files. Some information on this can be found in the WinHlp Wikipedia article. However, WinHlp32 is probably no riskier than other executables, and the only other recourse for abandoned programs may be to decompile the .hlp files and recompile as chm which has a number of limitations also.
Information on the files included in the zipfile is given below. The MD5 and SHA values provided in this description can be validated online at various locations.
Zipfile information: Contains WinHlp32.exe , winhlp32.exe.mui, and install.cmd . The data for the two binary files are as follows:
WinHlp32.exe file size on disk: 296,960   Version: 50.0.6000.16387   02/19/2007
WinHlp32.exe.mui file size on disk: 31,744
Obviously Komeil's CMD file can be read directly as simple text. Incidentally, these codes were generated with QuickHash 2.6.7.
The latest version of QuickHash is available here:   Hashcodes