|OK. Do it.|
If it is a desktop the just open it and connect it. If it is a laptop then you will need an external hard drive case to use it externally (will work with a desktop also). You need to know if this drive as well as the original hard drive are SATA or PATA (IDE) drives or one of each and you need to know if your motherboard supports the other type if you have one different than the original one. You will also need an available power connector of the correct type and a signal cable (SATA) or an open connector on an IDE ribbon (PATA)(note: you will need to set the correct jumper settings for this set up).
You have to be a little bit crazy to keep you from going insane.