1. Just start a new install with Centos and it will ask what areas you want to use/overwrite. No need to 'uninstall' Oracle, but do pay close attention to those qns, so you don't hose your MS.
Given you have successfully got a dual boot system, you already know what to do.
2. You'd have to show us exactly what cmds you tried (and msgs recvd) to use CD/usb before we can help you.