Associate file extensions with a program in Windows Vista (File Type Associations)
Our previous tutorial showed you how to easily set default programs in Windows Vista. This tutorial explains how to go a level deeper, and individually assign handler programs to file types (by file extension). The file extension determines the type of the file in Windows; an MP3 file is called fileName.mp3; a Word document is called document.doc or document.docx, etc. (Note that, by default, Windows Vista hides the extension for known file types, so you may not be able to see the file's extension without looking at the file's properties.)
Setting default applications to handle file types in Windows Vista
To manage file types and file type associations in Windows Vista, open the Start Menu, and click Control Panel. Then, locate and click on Programs: 
Associate a file type or protocol with a specific program
Under Default Programs, click on the "Make a file type always open in a specific program" link. In the next screen, Windows Vista loads the file extensions of all known file types.
These file extensions are either known by Windows Vista from the start, as is the case with common file types like MP3 and DOC, or file types that have been introduced and properly registered by third party applications you installed on your computer.
File Extensions Listing and Protocols
Windows Vista displays three columns of information for your file types/file extensions. Notice that values in the Name column start with a period: this is the period that separates the file name from the file extension. Clicking on the Name, Description, or Current Default column will sort alpha-numerically by the values contained in the column; click a second time to reverse the sort order.
The Name column contains the file extension itself, most often three characters long after the period; the icon associated with the file type is generally assigned by the operating system by default (Windows Vista), or by the application that registered the file extension. (A later tutorial will explain how you can customize the icon of select file types.)
The Description column indicates the type of file, when it was registered properly; the Current Default column indicates which program is currently set as the default handler for the particular file extension.
The first series of file types are Extensions; the second series of "file types" are "Protocols". Links to the iTunes Store, for example, are registered as a protocol: they load in your browser, which passes them on to iTunes.
Associate a program with a file type (file extension)
To change the program association of a file type, select a file type, and click the Change Program button to launch the Open With window: 
You can select one of the applications Windows Vista suggests (including the current default file handler), and click OK to create or confirm the association. If the program you want to set as the default handler for the file type/file extension is not listed, click the Browse button, choose the application you want to handle the file type, and click OK.
And you are done: you just associated the file type with the application you wanted as default. Also notice that Windows Vista displays this "Open With" dialog when it encounters a file extension that has never been registered; the principle is identical, Vista just forces the question if needed.
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