Your battery is probably soldered to your motherboard so unless you are confident with circuit board soldering, I would leave well alone.
I use a neat little program called "ClockWise" by RJ Software. It runs in the background and updates the computer clock to an atomic clock everytime I log on. It also has various other utilities like tracking how long you spend on the internet, alarm functions, calendar, sunset/sunrise/time zones etc.
I think Windows xp also has a clock update function somewhere.