WARNING on old CMOS Batteries!
If you suspect that your CMOS battery on the motherboard is dead, or may be leaking, remove it as soon as you can! The acid in the battery can and will damage the motherboard, and may render it completely useless!
All you need to do to remove the battery is use a small pair of cutters, and CAREFULLY cut the battery off of the board, preferably where the legs go into the board. Be sure to leave a small part of the legs visible so they can easily be removed when you install a new battery. Be careful so you don't damage the board.
Some later models may use a button cell battery that is in a holder. Simply pop the battery out and replace it with a fresh one!
If there is battery acid present on the board, It is advised to clean the corrosion off as best as you can, as if it sits too long, it will damage traces and solder joints on the board, with a 50/50 chance of it being unrepairable!