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After installing Adobe PDF reader: *.PDF will be opened by the same. *.html with web browser. *.txt with notepad. If we want to make notepad++ a default editor of text files. I guess it simply changes the filetype - program association.

Where exactly in the registry are these filetype - program associations stored in the registry? In other word, how does Windows know with which software/program it must open a certain file(type)

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up vote 9 down vote accepted

The open with associations are all stored in HKEY_CLASSES_ROOT.

This is a special registry hive that combines the local user's associations in HKEY_CURRENT_USER\Software\Classes with the system's associations in HKEY_LOCAL_MACHINE\Software\Classes.

Here's a shot from my registry for the PDF association you mention in your question:

PDF file association in the registry

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Just to extend Gareth's answer:

From playing with the registry it looks like

  • entries from HKEY_CURRENT_USER\Software\Classes override entries from HKEY_LOCAL_MACHINE\Software\Classes in the HKEY_CLASSES_ROOT\Software\Classes
  • setting file's extention association from command line using assoc command acts upon HKEY_LOCAL_MACHINE\Software\Classes
  • setting file type association from command line using ftype command acts upon HKEY_LOCAL_MACHINE\Software\Classes
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While the answers are (probably, I didn't check) correct, the sane way to modify file associations is with the GUI Microsoft designed for it. (That is, if you are a user and not a setup developer.)

Here is one tutorial with nice pictures, I found it with google:

But you can just open start (win7 menu or win8 screen, doesn't matter) and type file association, enter and it should open the correct control panel link. (Control Panel\Programs\Default Programs\Set Associations).

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