The Microsoft Windows Operating Systems use the AUTORUN.INF file from removable drives in order to know which actions to perform when a new external storage device (USB drive or CD/DVD) is inserted into the PC. The AUTORUN.INF file is a configuration file that is normally located in the root directory of removable media and contains, among other things, a reference to the icon that will be shown associated to the removable drive or volume, a description of its content and also the possibility to define a program which should be executed automatically when the unit is mounted.
This feature has been widely criticized by the security community becuase it is used by malware in order to spread by infecting as soon as a new drive is inserted in a computer. The malware achieves this by copying a malicious executable in the drive and modifying the AUTORUN.INF file so that Windows opens the malicious file silently as soon as the drive is mounted. The most recent examples of this are the W32/Sality, W32/Virutas and also the W32/Conficker worm which, in addition to spreading via a vulnerability and network shares, also spreads via USB drives.