Laptop BIOS battery

My friend has a old laptop (just about runs windows 98) but every time it starts up it asks her for the time and date. I am assuming this is due to the mainboard battery being dead, if its the same as a PC!?. Can I just buy one of those little round battery’s for it? and what’s the most common way of getting into older laptops? — I don’t want to break it :-/