An older machine is probably using APM (rather than the newer ACPI), so you will need to enable the APM module at boot to be able to control the power supply.
You can test this by running the command "modprobe apm" and then trying to shutdown. If that fixes the problem, you can make the change permanent by changing the following section of /etc/rc.d/rc.modules by removing the "#" before /sbin:
Code:
#### APM support ###
# APM is a BIOS specification for saving power using several different
# techniques. This is mostly useful for battery powered laptops.
#/sbin/modprobe apm