I'm on a Quad-Core Intel Xeon Mac Pro. I did all the upgrades for Leopard and currently running at 10.5.8. I purchased the Snow Leopard 10.6.3 version installation DVD but it doesn't let me upgrade. My optical drive works fine but when I put the DVd in for Snow Leopard, after a few minutes the system pushes the DVd out and does not show up on the desktop. I tried to start up with the DVd also, holding down the C and got the same result, after a few minutes the DVD is out and no installation. What could be the problem? What can I do to install? Thank you for your help in advance.