USB Defender is a portable, extremely easy in use program that vaccinate your drive against viruses.
It has two buttons: “Protect" and "Unprotect". First ‘Protect’ button you are using to vaccinate your drive, second - to remove protection. Just select a drive connected your computer and click ‘Protect’ button to vaccinate it.
USB Defender works well on Windows 7, Vista and XP. You can download it here.