You can get into your setup without a boot cd. When the grub menu starts hit the key that will let you add additional arguements to the kernel. I believe the key is 'a'. You can then add the word 'single' to the line and you will enter into single mode.
If you were booting into graphical mode go to /etc/rc5.d/ after you successfully boot into single user mode. Look for a file with the word 'iptables' in it. It should be something like S92iptables. Rename the file to K92iptables (using whatever number is used in FC3).
mv S92iptables K92iptables
If you type 'exit' you should boot into run level 5 and then you can make changes to your firewall rules. Or you can just reboot. Don't forget to rename the file back to what it used to be when you are done.