ClickOnce Installation Problem on Vista

We have a Winforms application published via ClickOnce, and I came up against a problem with the ClickOnce installation mechanism on my machine (Vista). Linking to the deploy manifest (the .application file) should launch the runtime to download and install the application. On my machine, this was triggering a dialog that prompted me to install the .NET framework (already available), and then proceeded to download the Winfx 3.0 Beta 2 runtime components.

The problem was that the wrong application had become associated with the .application extension – in my case, notepad2. To resolve, click Start > Default Programs > Associate a file type or protocol with a program, then find the .application extension. The current default handler should be set to ‘Application Deployment Support Library’.

In my case making this change then caused the correct behavior when clicking on a link to a .application file. I’m not sure quite how the default changed so easily, but think this was opening the deploy manifest in notepad2, and leaving the checkbox checked to use this application as the default.