Seriously you need a newer PC. Your friend doesnt sound like someone who knows much about computers so i doubt he built this pc 10 years ago when it was obviously built so i doubt you can just add another cpu. Sure you could add some more ram but to what end? Websites are getting ever more cluttered and resource intensive, youtube videos are probably unplayable on a machine of that type. There is no getting around it. No operating system out there is going to make a huge difference in usability for that system. Linux can be a pain to learn for the first time especially those small distros that rely on greater user knowledge to do things.
My advice is deal with a slow pc for a while and save up $100 or so and get an old core 2 duo machine. Doesnt matter which one as long as it works because anything will be better than what he has currently. Go find a business that is getting rid of their old PCs and buy one from them for cheap. Or find an old laptop or desktop on ebay etc etc. Spending any money on this pc is just money wasted.
Long Live SHIO!
