Download the Fedora disk image from a mirror:

# wget http://mirrors.dotsrc.org/fedora/5/i386/os/images/diskboot.img

Make sure that the USB device is unmounted:

# sudo umount /dev/sda1

Copy the img to the usb key device:

# dd if=diskboot.img of=/dev/sda

Reboot and then select a network based install by typing:

linux askmethod