Forget Leopard try Snow Leopard, or forget updating all together.. just boot the DVD from start up and try formating the HD and installing Lion...

If it doesnt boot the Lion disc.... am not sure, it might be the DVD wasnt burned properly?... try an external DVD reader if you got one? always had problems with SuperDrives.... so I took mine off and added an extra SSD, I use an external disc drive to burn and load Dvds....