When you plug in a USB flash drive in your PC with Windows Vista or Windows XP, it will seek for "autorun.inf" and run an executable file. Sometimes it's safe, but you cannot be sure that an executable file is not a malicious software. You can disable autorun, but what happens when you doubleclick the removable disk? Your guess is right, the executable file will run immediately. If you want to prevent "autorun.inf" from running from any USB flash drive, just create and start disable.reg:

REGEDIT4
[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\IniFileMapping\Autorun.inf] @="@SYS:DoesNotExist"

    Windows may cache AutoRun information from connected devices. The impact of this feature is that even after disabling AutoRun as described above, you may still experience AutoRun behaviors for devices that have been connected to the computer in the past. For this reason remove this cache by deleting the MountPoints2 registry key for each user:

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\MountPoints2