Hp

i have been working with this laptop for a few days and it's been VERY slow. it's not that it is filled with spyware and viruses that keep the process or running at full it's that the hard drive can't seem to keep up. After i load a program once it seems to be fine since it is in cache but it take windows quite a long time to load up and everything is just so slow to load. I take it with that message the hard drive is gona die and soon? how hard is it to replace a HDD in one of these laptops? I'v never done it and well this is a clients computer and i'd rather not make it not work at all. Would it have anything to do with the under fan not working? i was gona take a look at that also but i don't want to mess it up anymore. What do you guys think?