Firstly make sure you motherboard is capable of booting a USB drive,
if not your computer won't be able to make use of this bootable USB.
You can change the boot priority in the BIOS.
Please look at Step 1 and Step 2 of this tutorial here: Clean Reinstall - Factory OEM Windows 7
The Win 7 install (SP1) ISO is there and also the tool to install it to a USB drive.
Then follow my first instructions.