I want to wipe my iMac clean and reload. I have an older iMac from mid 2007. It came with Leopard and I have upgraded to Snow Leopard, Lion, and now to Yosemite. What is the easiest way to do this without having to start with Leopard, then install Snow Leopard, etc. ? Lion and Yosemite where installed from the Mac store. I only have dvd's for Leopard and Snow Leopard. Thoughts?