Macintosh does not shut down

Hi,
We have one Mac in the office used for designing labels and other artwork and the really funny thing is that it will not shutdown. Rather, it just restarts. I'm not a Mac specialist so, I don't know what to make of it. There is a NIC in it but it doesn't support wake. The 'soft' shutdown just reboots the machine. the 'hard' shutdown, shuts down the computer, but after less then a minute, it boots back up again.

Any help is greatly appreciated.

Thanks.

tk