Since the life of the 3 volt motherboard battery is so much better than it was a few years ago, I don't often need to replace them. However, a client called today and said her PC had been starting with the wrong year after a cold reboot for a few weeks and now it wouldn't boot at all. The boot process froze showing a checksum error and offering an F2 option to load defaults - this didn't actually work. I replaced the battery and now it's working fine.