MacBook won't let me log in, PLEASE HELP

I have a Macbook running OS X Leopard. I've been using it for years now. It always automatically logs me in. But a few days ago, when I turned it on, it took me to the log in screen. So i put in my password and it loads for a few seconds then the window buzzes and takes me back to the log in screen. I know for a fact that i am putting in the right password but it still won't log me in. I even used the command + S trick to change my password in terminal but it still won't log me in with the new password. I also tried the Command + S trick where you trick your computer to think its brand new and created a new user. butt once i do, it takes me back to the log in screen where i try to log in with the new user but still the same old thing. won't let me log in. I tried to Factory reset but when i press Command + R at start up, i only see my macbook HD and not my recovery HD. What do i do? Please help!