ISO is a virtual-disc format. ISO files are backups of real, physical CDs and DVDs. You can create a writable ISO, that is, one you can then burn to a blank CD to create a physical backup copy. You can do this by using the CD you want to backup as well as a virtual CD program.
Download and install Power ISO (see Resources). Insert the CD you want to get an ISO from. Click "Start" and select "My Computer." Right-click on the CD that's listed there. Select "Power ISO" and then click "Make Image" to create a writable ISO.
Download and install Daemon Tools Lite (see Resources). Click the "Make Disc Image" button. Insert the CD you want. Select the disc you want from the drop-down at the top. Click "Start" to create the ISO.
Download and install Magic ISO Maker (see Resources). Insert the CD you want. Click on the "Tools" menu. Select "Make ISO from CD/DVD-ROM." Select the Source drive in the new window, and then click "OK" at the bottom to create the image.