umm, I'm fairly sure that in order to make a bootable USB you need to reformat it. Although you can try putting a boot.ini that points to the install file in the root.
the installation goes as follows, get your install media ready (ie create a bootable USB flash drive with win7 install on it)
put your new rig toghether
start up, go to bios, make sure USB boot is enabled, plug in the USB, restart
wait for windows install to load, NOTE: IT WILL FORMAT THE DRIVE THAT YOU CHOSE TO INSTALL WIN7 TO.
so make sure you back up all you data