To make sure that your BIOS/UEFI is configured to boot from a USB drive, follow our Boot from USB drive guide. The USB drive you want to make bootable should have at least 4 GB in available size. If you have the original Windows installation disc, you can use that instead of the ISO image.
4.2 If you have the installation disc (DVD).4.1.2 Using Windows 7 USB/DVD Download Tool.