I have the same problem I thik and this didn't help. On one of my machines, when I would right click a file and select "TextPad" from the menu, it would launch TextPad but not load the file. I had to drag the file onto Textpad after it was running.
First, I uninstalled and reinstalled TextPad.
Then I tried what you suggested.
Now things are worse. Or maybe it was always this way before and I didn't notice it. Anyway, TextPad still behaves the same way, but now when I select "Open With", TextPad isn't in the list, so I click [Browse] and select TtextPad.exe. But it still won't show up in the list.
Ok, this is a bit of a hack, but it worked for me to get the excellent VirtualDub back into the Open With list:
Th idea is to uninstal the program, and then manually edit out of the windows Registry the Keys to the program that won't work (the TextPad.exe folders, problably inside the 'Applications' folder (or something like that), there may be multiple one's, and possibly one's with a slightly different name)
Obviously, this is dangerous, so let's not go deleting anything we shouldn't....
Use the search engine
Other solutions are welcome, all I know is that it worked...