Changing the battery on your motherboard couldn't be more simple.
First rule is to have a look at where it sits - usually in a corner away from the high voltage end and away from the PCI and AGP slots. Its usually about the size of between a 20p and 10p coin. Identify its number, then pop into your local pooter shop - or any other outlet.
WITHOUT any power on!!!!!!! lift the holding spring take one out and pop the other one in. Tis done!
PPP