I use to have this problem with my Network card, very weird. The BIOS wake on LAN was enable and everytime I shut off the computer it would restart instead. So I just disable that wake on LAN feature on the BIOS and NIC card under device manager and the problem went away. It migh not be the solution but just an idea for you
Because I added another NIC card to the PCI slot, my original one was buildin. The PCI NIC card enable another PC from the network to wake the computer up remotely. So when I started having this issue when I try to shut down I immediately know it was the NIC because it was the first time I had the problem when I just added this hardware.
Anyways, hey quiq have you tried updating your BIOS?
I could never figure out why it does this. But I know that the features keep the NIC card power on while the computer is off, that's why sometime the NIC light is still on at the back of your computer while your computer is off. Because the NIC must be able to recieve this "Magic Packet" from a remote computer to wake it up, it can't do this if it's at the off state.
Maybe a bug in the design, because it shouldn't restart the computer if you are shutting it off. Could be the Operating system itself, a bug within the power management.
I have this problem. It restarts when I select shut down, and then when I push the power button on my case, it restarts again. I have to disconnect the power cable for it to stay off. If I put the power cable in, it automatically starts up.