WinXP won't let me use an application to open a file ?!

I can't get WinXP to allow me to open a file using a selected
application.

I want to use Scintilla's Sc1.exe to open several of the files that
used to be associated with it.

for example
Matlab script program
trythis.m
used to open with Sc1.exe, but now WinXP won't let me open it that
way.

I can still open the Sc1.exe application, then find the file and open
the file. But why can't I simply double click like I used to?

I went to tools options and deleted the .m tie. Now I must choose
what application opens it, but...

So I click to open the .m file and get the browse, find Sc1.exe, but
nothing happens, it won't open

WinXP thrusts me back into using a choice of Notepad or Wordpad.

How do I get control of this #%!@^#$* system again? and tie opening
any file to any application I choose?

Robert