Hi,
There will be a small cell in every motherboard. This supplies power to the clock (hardware) while the computer is turned off. If this cell becomes weak, time lags. This cell costs approximately Rs.15 or 20 ( i dont know exactly ). It looks like a wrist watch cell, but bigger in size. Position of this cell varies from one motherboard model to other.
Take a look at this pic :-
*img452.imageshack.us/img452/589/asusp4c80023xw.th.gif