The battery only saves the CMOS settings on the computer and keeps the clock running when the computer is disconnected from the mains. It doesn't have any effect on how Windows runs.
The battery is a silver disk about an inch in diameter. If you can tell us what motherboard your computer uses, we may be able to point you to a picture of the battery on the motherboard. I think it might be this one. click here. If I'm right, the battery is in the foreground at the centre.