PowerMac G5 wont close down

I have a Powermac G5 duel core and when I try to shut it down all that happens is that all the icons disappear then it freeezes with just the background pic showing. The only way I can turn off from there to unplug the machine, which is a pain.

Can I revert to last weeks setup (as with windows) or is there any other solution.

Got the original install discs that came with the machine? If so run Apple Hardware Test which will check out the logic board, amongst other things. If not boot from your current system disc, go into utilities and run Repair Disk from there. You have to be booted from the disc for this to be available.