Thank you Fornhamfred.
I tried running an OpenSuse live disk and it had the same problem. I also tried typing "shutdown -h now" in the command line. The splash screen comes on, the line progresses, the numlock, capslock, & scrolllock lights flash, then nothing. Power light is still on, disk is still spinning, fans still running. Restart works fine.
This is really baffling. I read somewhere about a possible corrupt swap partition, but have been unable to find that thread again.
It also seems a lot of people in Felicia and Ibex are struggling with the same problem.
Any ideas will be greatly appreciated, as I'm currently shutting down by rebooting to XP and successfully shutting down from there.
Also, I am the only profile on the machine, so there should not be a conflict with other users on the network, especially as I don't even have Samba installed. I have also tried disabling the Network Manager, but still had the same problem.