After you create your bootable Win 7 USB installer, you should plug it into an available USB port. Power up the laptop and go straight to BIOS, boot device options and set the USB drive as your primary boot device with your HDD as the secondary boot device.
Save & Exit and the after the reboot you should have the installer intialize.
You should know that ISO's for Win 7 are non existent on the www(free of malware, that is).