I have a little trouble with erasing the hard disk on my mac pro. First off, according to the procedure, I inserted the OSX 10.5 into my macpro before restarting, then I hold the C button which suppose to boot to the installation interface where I can go to disk utility and erase the disk, but instead, after holding C, it took a while to load and there's a message telling me to restart the computer. I did what the instruction told me, so I hold the power button until the it shuts down, then I push the button again to restart, but the same message came up telling me to restart. I have done this over and over again, and it doesn't seem working at all.