Changing the drive is a matters of a few crosshead screws. The problem then is the Operating System. Is the existing drive working? If so, it's possible to clone that drive to a new one in an external caddy. If it's not working, you'll have to get/ borrow a DVD of the same OS as is presently installed. You will also have to be able to read the activation code on the sticker underneath (sometimes this gets rubbed off, and that makes life very difficult).
WPA2 hack: How secure is your Wi-Fi?