Windows 7 - How to change the default double click action?

I knew how to change this in XP, but I just can't work it out in 7. For an example of what I mean, in XP you could change it so that for .reg files double clicking would Edit and you had to right click to Merge. That was Microsoft's own example for a 'How to' for XP. I'm completely stumped though...

Are you looking for changing default program to open a file, such as AVI .... change the open option on double click from WMP to VLC ?

No no, I want to change which one is selected as the default action (the one in bold) that is performed when you double click. It was really easy in XP, in fact while searching for a way to do it in Windows 7 I found a guide for XP here. I hope that'll help to explain what I'm trying to do.

This page has an interesting method. Take a look.

Windows 7 Help: Change the Default Double-Click Action? - Icrontic Forums

Thanks. I tried this with the example (batch files) and it worked great, but I couldn't get it to work with my intended option. I think it's because it's an option added as a result of a shell extension, it's not actually possible to do what I was trying to do. Thanks for your help though.