My cousin has recently purchased a used Macbook from some online auction site (she can't remember, but knows it's not eBay) for less than 300 dollars. It came without a power cord, install discs, or even Mac OS on it. The previous owner had somehow installed Windows XP on it and erased most of the Mac info on it. I'm told that I'd need the exact version of Mac OS to reinstall, but we can't seem to figure that out. Only solution I can see (apart from purchasing a new one from apple.com and chalking this up to careless internet buying on her part) is to replace the harddrive. Will this fix the problem or is there anything else we can do?