When you put a disc into the PC it sometimes runs a program or brings up a menu asking what you want to do with the files on the disc. This feature is known as Autoplay and it controls what happens when a CD, floppy disk, USB key or portable hard disk is inserted into the computer. If the disk or device contains certain types of file, these will be automatically started when it is inserted.
To temporarily stop this from happening, simply hold down the Shift key on the keyboard at the same time as you are inserting the disc. Release it once the light on the front of the drive has stopped flashing.