Microsoft has dropped support for legacy help files with .HLP file extension in support for newer .CHM files. As a result, you cannot open the .HLP files on Windows Vista. If you double click on an .HLP file in Windows Vista, you would see an Open with… dialog instead of the actual help file.
If you have an older software that uses legacy .HLP files, or you want to use any .HLP file, then you can add support for them by installing a downloadable patch from Microsoft. In the knowledge base article KB917607 Microsoft has detailed how the support for .HLP files has been dropped and how you can install a Windows 7 version of WinHlp32.exe.
The WinHlp32.exe program can be downloaded from Microsoft Download Center. The download requires a genuine Windows validation, or users can download using the following direct download links shown below :
For Windows Vista 32-bit : Windows6.0-KB917607-x86.msu
For Windows Vista 64-bit : Windows6.0-KB917607-x86.msu