Whilst the mouse and keyboard unit are wireless and will broadcast at the same frequency whether you turn the machine off at the mains or not, it seems your reciever modules that are plugged either into a usb slot or into the PS2 connectors need power to retain there recieving settings. When you shut off the mains power they will lose the frequency settings that your wireless devices are transmitting on and presumably revert to a default. If you power you machine down and turn the power supply off with the switch at the back rather that at the wall you should eliminate this problem.