possibly the CMOS battery on your motherboard, if you open the case you should see like a big watch battery, flick it out and stall near doors leading out of the market to bus station should be able to supply.
Just switch it off, leave the power lead plugged in, but turn of at mains, this will ground it for you, if you stay away from power supply would be ok even if plugged in only 5v goes to board.
