 <p>Add the protocol (<code>http://</code> or <code>https://</code> for example), then the handler knows what to do:</p> <pre><code>System.Diagnostics.Process.Start("http://google.com"); </code></pre> <p>Windows checks the file extension list, and that included protocols too. There it finds <code>http</code> maps to your browser. You can consider to be 'lucky' it detects <code>www.</code> too, but I wouldn't depend on it too much.</p>