perhaps it is outtimed, but you can boot your client with your first redhat cd.
You will find yourselve logged as root into a Linux, that has your old filesystem mounted at /mnt/sysimage.
There you can manually reset the system-auth.
I read somewhere, that any bootable Linux-CD / disc / ... would do this job. Knoppix, or whatever you can find...
Clemens von Musil