Having trouble getting windows XP to consider my self-made app as default viewer

Factual Questions
1

I have an application which I built to be the default viewer of a certain file type.

When I go to tools->folder options->file types, find the file type, click ‘change’ then click ‘browse’ to find the app. I double click the app and then windows blatantly ignores the app I chose and stays with what it was previously using. Not so much as a message saying ‘This application is not verified to view this file’ or ‘this app is not safe’ or whatever.

2

I figured it out.

I clicked ‘restore’
which changed (the button) to ‘advanced’. From there I was able to create a new action for the file. I created a new action called ‘Open’, used the custom app, made it the default action

3

For future reference, all that sort of information is stored in your registry under HKEY_CLASSES_ROOT.