Reinstall 10.4, now can't install 10.5

My '07 MacBook Pro, OS X 10.4.11 was running so slowly that I figured I would try a fresh-install of my os as I had often performed with my PC, but that simply isn't the case...

Because I started a process that is irreversible (going from 10.5 to 10.4.11), I can perform no more necessary upgrades due to the fact my CD/DVD is no longer capable of reading from this drive.

At any rate, if this now makes better sense, is there any direction you can offer me? Thanks