Do you want to format it or re-install Windows? You can download some utilities that will boot of a USB drive to format a disk. http://www.ultimatebootcd.com/customize.html,
you can make a bootable USB drive using this.
If you want to re-install Windows 7, did you check if the vendor provided a recovery partition on the hard drive? Yyou should see an option to restore when you bootup. If not, this will help you http://www.ehow.com/how_5250777_install-flash-drive-usb-stick.html