Disk error trying to boot with USB w/Windows Defender Offline

Trying to clear my mom's computer of some viruses. She has Microsoft Security Essentials and it said I needed to download Windows Defender Offline.

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I downloaded to my personal laptop and created the files on a brand new clean USB flash drive.

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Went through the steps necessary through BIOS to move the USB drive to top of the Boot list.

When I put the USB in and restart the computer, it gives me a Disk Error message.

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Any ideas what the issue could be?

Computer I'm trying to clean is a desktop Dell Dimension.

Source: http://www.bleepingcomputer.com/forums/t/502918/disk-error-trying-to-boot-with-usb-wwindows-defender-offline/

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