Lifehacker links to a free utility from Microsoft that will mount an ISO image for you. Although other utilities are available to do this, they usually either cost money or infect your computer with malware. You have to create the ISOs yourself (just use a program such as Roxio), and then this utility will mount it. A good use would be, for example, at a LAN party where you only have one copy of the game CD but each multiplayer needs a copy to run the game.