Iocell's VirusDrive seems like a good idea; it's a USB flash drive with built-in auto-updating antivirus software in a hidden partition. Of course, if you're willing to roll your own, you can go Iocell one better and make a bootable flash drive that you can use as an all-around recovery and emergency tool, but for general paranoid users who are worried that a virus might have corrupted the antivirus software on their hard drive, this could still come in handy some stormy day.