The original poster wasn't talking about older PCs not being able to shut themselves down. This is a hang on shutdown, so the PC never gets to the stage of switching off or saying "it is safe to".
This problem was common with newer pcs, not older ones...
Which problem are you talking about, the first, on-topic comment or the later off-topic comment?
Either way...
The first problem can happen if a system is lacking in any of the four respects I described. The second comment, describing a possible solution is
only for older PCs.
PCs with ACPI enabled and properly working will be detected by Windows (98 and later) and will power off themselves when sent the shutdown command. PCs without ACPI, the older variety, don't do this by default, the shutdown command doesn't make the right requests by default, so on NTx based systems, that "APM/NT Legacy Power Node" needs to be installed by the user. Or the PC supplier may have done it for you.