Microsoft Knowledge Base Article 929149 contains the following INTRODUCTION:

With Windows XP we took steps to make it easier for customers to find software applications that can operate on specific file types. To do this Windows XP added a web service which presents applications which can operate on a given file type. An Example of what this may look like for a .DOC file is here:

http://shell.windows.com/fileassoc/0409/xml/redir.asp?EXT=doc

With the release of Windows Vista, we have improved the existing service to ensure the best possible customer experience.

Microsoft has re-designed the system to provide the user with a broader selection and allow them to choose an application more efficiently. One of the methods will be to have direct links to the download or purchase offering as often as possible.

More details on what is required to add an application to the File Association service are below.