I had the same problem, not every time, but when it happens it's a pain. sometimes it would restart 2 or 3 times before I killed the mains.I thought it might have something to do with the fact that I'm powered through a UPS. I was living with it, and turning off at the mains when the restart happened. Recently while doing something else I noticed that in the BIOS my boot preferences were set to CD - HDD - Other. With the 'Other' boot disabled the problem hasn't re-occurred. Re-enable 'Other' and the problem is back. Naturally the boot menu and the motherboard manual give no clues as to what 'Other' means.
My motherboard is an MSI G41M-P28 with American Megatrends Inc bios
It's the printer! Printer on - won't shut down. Mains light on start button goes off at shutdown, flashes twice at 2 second intervals and machine boots up again, Printer off no problems. It's a Canon ip4700 -CUPS+Gutenprint v5 2.8-pre1 if that's significant.