Unrecognized file type

Question

We have a custom program that runs on xp. One of the quirks is renaming a file specific to the program. This file has always been an unrecognized file type (which is ok). I accidently changed the file extension to .txt and specified excel as the program to open it with. I need to change the file extension back to what it was and not have excel open it. No matter what I do I cannot disassociate excel from this file no matter what extension I give this file.Any clues as how to "go back in time"? BTW this file is on a network and when accessing it from another computer, everything shows the way it should be, it's only when it's opened on my computer does it show the problem.Any help is appreciated.