[CentOS] Shutdown/poweroff does not power off machine

[Michael Klinosky]

m.roth at 5-cent.us wrote:
> Must be the Optiplex - Thursday afternoon, we just did poweroffs on well
> over 100 servers (HVAC maintenance this past Sat!), and it worked just
> fine.

I have the same issue with one of my boxes. 'Shutdown' will get to the 
point of actually powering down, but just sit at 'System halted'.

Here are the specs:
mb: Gigabyte GA-7ZX rev 1.0
cs: VIA KT133/686A
cpu: AMD Athlon 1.33 GHz
socket: A/462
os: CentOS 5.3
RAM: 384 MB
video: GeForce4 MX440

I recently bought it (used), and this is the only OS I've used on it 
(not even a previous CentOS).

What can I check? (logs? ...) What should I look for?