If you already have Linux installed, it is possible to boot the Fedora installer to do installs or upgrades without a boot CD.
In the images/pxeboot directory of the distribution repository, there is vmlinuz kernel and initrd.img ramdisk image. Download those and put them in /boot. Then edit /boot/grub/grub.conf and add the following entry:
title Fedora Installer
kernel /vmlinuz
initrd /initrd.img
This assumes that you have a separate /boot partition. If not, add /boot to the paths.
Then, reboot your machine and select the “Fedora Installer” entry from the grub menu. Once the installer has booted, you can do a network or hard disk install or upgrade.