By way of Omar Shahine comes this excellent resource:
Windows XP File Association Fixes by Doug Knox

If your file associations in Windows XP ever get wonky, the .reg files provided by Doug Knox will allow you to quickly and easily put them back to their Windows default settings:

“The files listed here are all ZIP files, which contain a REG (Registry) file. Download the ZIP and open it.  Extract the REG file to your hard disk and double click it.  Answer yes to the import prompt.  REG files can be viewed in Notepad.  Each of the REG files contains the default settings for the file extension indicated.”