LOL, when you associate a extension (p.e. .asd) to a software (p.e. Photoshop) Windows always try to open the file with the soft... that's how it works...
Go to Tools > Folder Options > View File Types, then search the extension and click on [Restore] or something. I'm just guessing, I have a spanish Windows.
I'm glad that changed to Notepad2, it's really easier to read.