You can launch the default browser to display HTML content using the Process.Start method in the System.Diagnostics namespace by specifying the URL as the argument. But I don't recommend this approach because sometimes the browser will launch behind your application.
A more reliable option is to use the Help.ShowHelp method to launch HTML content (even if the content is not on-line help). Call Help.ShowHelp with the active control as argument 1 (or use Form.Active Form). Specify the URL as argument 2, and be sure to include the prefix (file:///, http://, https://, etc).