When the worm runs on your computer, it enumerates all drives of your PC until a mapped drive is found. The worm tries to copy itself to the mapped drive. Worm:Win32/Autorun then writes an autorun configuration file named 'autorun.inf' pointing to the worm executable.
When the removable or networked drive is accessed from a computer supporting the Autorun feature, the malware is launched automatically.
This threat tries to use the Windows Autorun function to spread via removable drives, like USB flash drives. You can disable Autorun to prevent worms from spreading: