Microsoft removed the legacy Windows Help .hlp file viewer, winhlp32.exe, from Windows Vista, Windows 7, Windows Server 2008 and 2008 R2.
Simply download and install the Windows Help viewer and the classic help functionality will be restored.
Windows Help (WinHlp32.exe) is a Help program that has been included with Microsoft Windows versions starting with the Microsoft Windows 3.1 operating system. However, the Windows Help program has not had a major update for many releases and no longer meets Microsoft’s standards. Therefore, starting with the release of Windows Vista and continuing in Windows 7, the Windows Help program will not ship as a feature of Windows. If you want to view 32-bit .hlp files, you must download and install the program (WinHlp32.exe) from the Microsoft Download Center.
Download Windows Help program (WinHlp32.exe) from here