wdzhou:
I give you my simple way to implement it. To power it on through 'Wake On LAN' (Wake-on-LAN - Wikipedia), and to power it off with a small service program which runs on PC and receives a specific UDP(or TCP) message then to shutdown the machine. All you have to do is setting your BIOS and OS on PC and writing a simple PC service program, and all you have to have is a network shield.With these, you can even control the power of your PC through Internet remotely.
I don't think this is something what would meet my requirements, the target is to "fully disconnect from mains" the whole system and not to put in sleep/wait mode.
Additionally I still need to send those UDP messages - I assume from arduino which would required using ethernet shield 
And last but not the least - I'm not really dealing in my project with "real" PC, it's NAS running on linux, this is quite specific, that device is not supporting wake-on-lan.
Nevertheless thanks for your thoughts !