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Select the archive and install option. As with any major work on your system, its always safest to backup your system prior to the work.
 
To start with you need a retail Mac OS X install 10.5 DVD to make the basic 10.5.x installation. Then you download the 10.5.8 combo updater from Apple and apply it to your basic install. A 10.5 installation disk from another Mac will not work as these OEM disks are machine specific.
 
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