While most Visual Studio commands have a name and you can execute them using DTE.ExecuteCommand, there are cases where commands don’t have a name, just the Guid and Id that truly identifies them, For example, the command to filter the Task List of Visual Studio 2005/2008 by its combobox (my next article will be about this). This article explains how to get the Guids and IDs of Visual Studio commands, and how to execute a command from the automation model (EnvDTE) given its Guid and Id:

HOWTO: Execute a command by Guid and Id from a Visual Studio add-in
http://www.mztools.com/articles/2008/MZ2008013.aspx