Reassign File Associations in Windows
Windows remembers which programs are associated with each file type, so when you double-click on a file in Explorer, Windows knows which program to launch. For example, if the file ends in .XLS, typically Microsoft Excel will be launched.
However, some file types may be assigned to many programs, such as .MP3 and .JPG and so applying a Service Pack or installing a new program may reassign the file associations, either back to the system defaults or away from the system defaults.
If you find that the a file is no longer being opened by the program of your choosing, the association can be restored easily:
Assigning File Types for Older Applications
Either by design or bug, some versions of Windows disallow assigning 16-bit applications through the procedure described above. This can be worked around manually, as follows:
Removing Custom Assignments
Windows can also store associations for file types on a per-user basis. These generally over-ride those set via the shell (Default Programs) or ASSOC/FTYPE. Unless there is a special reason, these should be removed. Use Registry Editor and browse to HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts and delete or rename the key for the desired extension.
Some programs have in-built functions to assign certain file types to themselves. For example:
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