I recently came across a fantastic tool that enables users to install fully functional and update -able antivirus software on their flash drives, external hard disks, ipods e.t.c.
It is free. It contains no spyware. There are no advertisements. It isn't a limited or trial version. There is no additional hardware or software to buy. You don't even have to give out your email address. It's 100% free to use, free to copy and free to share.

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www.portableapps.com