A lot of dust in the fan can make it spin slowly, causing the computer to overheat over time. Open up the laptop (remove its keyboard) and take a look at the fan. If it seems to be dirty (or even if it's not, you might as well while you're there), clean the fan and make sure there isn't a buildup up dust on it, as this can cause the fan to slow down significantly. Once it's clean, double check the fan's connection to the computer itself (this is set of multicolored cables near to the fan that connect to the motherboard). Reconnect the keyboard and restart the computer. If it still overheats, then the fan is likely faulty.