In the flash disc exposed to the virus usually (not always) there autorun.info file that contains commands to run the virus program. Well, when the flash disc is inserted into the USB drive its autorun feature not be disabled then Windows will automatically run programs at background without a confirmation to the user .
1.Click the Start button - Run.
2.Type gpedit.msc and press OK.
3.Click on User Configuration - Administrative Templates - System .
4.Click 2x on the Turn Off Autoplay .
5.Click on the option Enable .
6.At the option Turn off Autoplay on , select All drives .