My problem was a bit different: i use pentium 3, and am able to shutdown normally on all distros i've tried, except (K)ubuntu. In kubuntu, when i chose to shutdown (even by shutdown -h now) i hear HDs shutting down and then the system stalls, i only get the usual kubuntu shutdown screen. I had to push the shutdown button on a pc to end the process.
I found this solution in one of the posts at http://kubuntuforums.net/, don't remember what exactly. I've seen that some had your poblem too, so take a look there.