why wrote:
One of the simplest solution is to check the application used to perform the open action for the coressponding file from "File Types", e.g.
"C:\WINNT\hh.exe" myChm.chm
"C:\Program Files\Adobe\Acrobat 6.0\Reader\AcroRd32.exe" myPdf.pdf
"C:\WINNT\system32\NOTEPAD.EXE" myTxt.txt