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To boot from the USB drive, you will have to set the BIOS to boot from USB first instead of the hard drive. To open the BIOS, reset your computer and hit the displayed key to enter Setup. The key varies by manufacturer, but is usually F2, F10, F12, or Del. Open the Boot menu in your BIOS. Change the 1st Boot Device to your USB drive. Be sure that it is inserted, or you may not be given an option to choose it. Depending on your manufacturer, it may say Removable Device or list your flash drive’s model. If you set the boot order correctly, your Windows 7 or Vista installation will load after the manufacturer’s logo disappears. The installation process will load and the Windows setup will begin. Check out our guide on  installing Windows 7 for an in-depth walkthrough.
Change the boot order. Save changes and reboot. Install Windows.