That loud fan is definite sign that there hasn't been any clneaing maintenance performed on your sister's old laptop. When dust builds up on the fan blades and other locations inside the laptop, it will cause the computer to overheat and get really hot, and possibly shut down to protect itself. You can usually use canned air ( available most places where computers and computer accessories are sold ) to blow air into the laptops vents on the sides, and force the dust/dirt out of a vent on the opposite side. If this maintenance isn't done regularly ( every few months ) then the dust begins to build up on the fan blades and throw it out of balance which usually makes it very noisy. This seems to be your problem. If you don't have any experience in opening up a laptop, I'd suggest either asking a friend that has this experience ( and proper tools ) for some help, or take it to a computer repair shop for the maintenance. It will cost you some money to have a repair shop do the work, but if you just use the canned air at a regular interval, you won't need to have the shop do this again. The fan blades are delicate and need to be treated with care if you or a friend decide to do the clneaing yourselves. Sometimes, depending on the brand of laptop, the fan is difficult to get to, so be wary of this. If you do take it to a computer repair shop, have them test the battery and charger while they have it to see if there's a problem with either.