I have an early 2007 model macbook pro and was finally able to upgrade to leopard. I cleaned out my hard drive so I had 15GB free space checked the processor and the memory and I have all of the requirements to install it. It still says can not be installed on this machine. I currently have OS X 10.4.11 installed.
That DVD originally shipped with a Mac and can't be used to install on another Mac model. If you don't have a retail Mac OS X 10.5 DVD lying around(these are all 10.5, 10.5.1, 10.5.4, or 10.5.6), buy Mac OS X 10.6 on DVD.