other wise to do it manually its in some init script, youll just have to look for it, someone else here might chime in on where it is.
or, you could "duct tape" the problem...
just put
ethtool -s eth0 speed 100 duplex full autoneg off
in a script and have that script run after the networking script has been run.
hint: you put the script in the /etc/rc.d directory, and symlinks to run the script in /etc/rc.d/rc$.d
where $ is the desired runlevel you want the script executed in.
edit: also, dont put it in the /etc/rc.d/network script, unless you know everything that script is doing.
one more thing, you dont need a symlink for that script to be run on shut down, only on start up, as the network script will take care of "shutting down" the networking settings.