Daemon Tools - very popular alternative, but remember to uncheck the box asking you to install secondary software with it! Due to a recent change you should mount the image file through Daemon's 'Mount to SCSI' option otherwise running the game without cd won't work.WinCDEmu - recommended, as it's completely free and doesn't install unwanted secondary software.If you don't own the game at all, download it here and follow the instructions below. The procedure will work even if you have RotWK already installed, regardless of your patch.
Fortunately, running the game without a disc is simple. This is useful for players who don't own a working disc or who just want to save time by not having to constantly insert it before starting the game. This guide explains how to play Rise of the Witch-king without a DVD.