navigator.userAgent

private string getDefaultBrowser()
{
    string browser = string.Empty;
    RegistryKey key = null;
    try
    {
        key = Registry.ClassesRoot.OpenSubKey(@"HTTP\shell\open\command", false);

        //trim off quotes
        browser = key.GetValue(null).ToString().ToLower().Replace("\"", "");
        if (!browser.EndsWith("exe"))
        {
            //get rid of everything after the ".exe"
            browser = browser.Substring(0, browser.LastIndexOf(".exe")+4);
        }
    }
    finally
    {
        if (key != null) key.Close();
    }
    return browser;
}