Any specific reason for downgrading?, because if that Mac was originally shipped with leopard, downgrading to tiger may render some hardware unsupported.
Anyway, if the data on that Mac is critical, backup the home folder to an external drive before attempting anything.
-Insert the Mac OS X 10.4 Tiger installation disc in your computer’s CD/DVD drive.
-When the Mac OS X Install DVD window opens, double-click Install Mac OS X.
-When prompted, click Restart and enter your password. Your computer should restart from the DVD.
-Follow the prompts that appear onscreen to select your language and advance through the screens until you get to the Select a Destination screen.
-Select your hard disk as the destination. It will probably appear with a red X, indicating that Tiger cannot be installed on that volume.
-Click the Options button.
-In the Options dialog that appears, select Archive and Install. (It may already be selected.) You will not be able to turn on the Preserve Users and Network Settings check box. Click OK.
-Click Continue and follow the remaining prompts to install Tiger on your computer.
-At the end of the installation, the computer restarts and displays the Mac OS X Setup Assistant screens. Follow the prompts to set up your computer. You will not be able to copy existing setup information from the Previous Systems folder that now resides on your hard disk or from a backup because those settings were created with a more advanced version of Mac OS than what is now installed.
In the Options dialog, you could also choose Erase and Install, but that would erase everything on you hard disk. Although this is always the best option for installing system software since it does a real “clean†installation, you must prepare by backing up all of your documents before doing the installation. Do not choose Erase and Install unless you’ve backed up your hard disk to another disk you can restore from.
