Windows does not offer an easy way to get rid of the OpenWith associations.  These are associations made when you double click on a file that has nothing associated with it.  This will open up Windows' OpenWith dialog box.  If you do not uncheck the "Always use this program to open these files" checkbox or click on Cancel, the selected program will forever be associated with that file type.

There are other ways that items get in the OpenWith list.  One is that programs can put them there.  Another is that some versions of Windows put every file run by another file in it.

This Editor will list those associations and allow you to undo them.  The entries that have (FileExts) on them are listed in a different part of the registry and will not show up as an Open With entry in Windows unless there are two for an extension, but in WinXP there are probably a lot of FileExts entries which are not needed and are littering up your registry.

Please note that you may not want to undo all of these associations.  So please look through the list and only undo the ones that you do not use or are no longer valid.  The decision is up to you.

All associations are not listed here.  Only the OpenWith associations.

The Extension and Program that is associated with that extension is listed in the listbox.  To undo one select it and press the Undo Association button.