Autoplay begins reading from a drive as soon as you insert media in the drive. As a result, the setup file of programs and the music on audio media start immediately. Prior to XP SP2, Autoplay is disabled by default on removable drives, such as the floppy disk drive (but not the CD-ROM drive), and on network drives. Starting with XP SP2, Autoplay is enabled for removable drives as well, including ZIP drives and some USB Mass Storage devices.
autoplay only begins when your system recognizes the device or it is being plugged into it previosuly, some devices need their drivers to be installed before using.