How to view old Help files in Windows Vista
But Microsoft decided to leave that application out of Windows Vista to encourage developers to use more up to date help file formats. There's just one problem. When you buy a new computer or a new operating system you don't buy all new software. So there's probably more than a few applications you run that were designed for Windows XP or earlier. And you can't read their help files using Windows Vista. Not out of the box, that is.
It turns out there are two pretty simple ways to add support for HLP files to Windows Vista. First up, you can download and install an "update" from Microsoft. It weighs in at a hefty 601KB, so I can see why the company left it out.
If you've got an XP machine lying around, or if you're running a dual boot system, you can also just copy WinHlp32.exe from the Windows directory. When putting it on your new machine, you should probably stick it in Program Files or another directory, not Windows. In order to associate HLP files with the application, you'll need to find an old one, right click it and select "open with" to select the default program.