This annoying and troublesome problem has been around far longer than Windows. Some programmers (who should know better) have set up their applications to change around your File Types Without Asking, so that double-clicking on an associated file launches their program. The problem is that any File Types that you had set up previously are ignored and overwritten. For example, say you had configured Windows to launch a certain graphics viewer every time you double clicked on a .GIF file, or you want to use a certain text editor with your .TXT files. Then, you decide to install a new program, and its installation program changes your File Types so that the new program is launched every time you double-click on a .GIF file. Or, worse yet, the File Types are changed every time the program is launched - a little arrogant, don't you think? For some applications that use proprietary file types (like .XLS for Excel files), this isn't a problem, but many applications use many common file types, and their programmers should be thoughtful of their users.
as of right now there is no perfect way to fix this problem but heres some things you can do to aleiviate the stress with getting it back to like it was should it hsppen to you...
Back up select file types:
Use the Export command in the Registry Editor (regedit.exe) to export select portions of HKEY_CLASSES_ROOT to one or more files.
Then, restore your backups by applying the registry patches as needed.
See the Annoyances books for details on the structure of file type information in the Registry.
On-the-fly Workaround (Windows XP/2000/Me):
Right-click on any file and select Open With... to choose a new program association on the fly.
On-the-fly Workaround (Windows 98/95):
Right-click on any file while holding the Shift key and select Open With... to choose a new program association on the fly.