It looks like Microsoft has dropped support for the ancient hlp format (WinHlp32.exe) under Windows 10 completely.
I have found a solution to install an older version of "WinHlp32.exe" under Windows 10:
1. Extract the file "WinHlp32.exe" from a Windows XP installation (Windows directory)
2. Copy the file "WinHlp32.exe" to your Windows 10 system in folder c:\temp
3. Start notepad and create a new file with the following content:
takeown /f "%windir%\winhlp32.exe" >nul
icacls "%windir%\winhlp32.exe" /grant *S-1-5-32-544:F >nul
copy /y "%crtpth%\winhlp32.exe" %windir%
icacls "%windir%\winhlp32.exe" /setowner "NT Service\TrustedInstaller" >nul
echo Press any key to Exit
4. Save the file as ""c:\temp\Win10-Hlp32-Fix.cmd"
5. Open a command prompt window as Administrator (very important)
6. Switch to the directory "c:\temp"
7. Start the batch file "Win10-Hlp32-Fix.cmd"
If all goes well you can use old .hlp files on your Windows 10 system afterwards. On my Windows 10 system it worked perfectly.
For a German translation of this tip please go to the German forum: http://community.pmail.com/forums/thread/45441.aspx
Team leader German Translation Team