ethtool or mii-tool are command line utilities to change a NICs settings.
I do not know if RH9 supports this option but you can try.
Add a line to the /etc/sysconfig/network-scripts/ifcfg-eth0 file like:
ETHTOOL_OPTS="autoneg off speed 100 duplex full"
If that fails you can add the ethtool or miitool command to the rc.local file.