how to reinstall os after hdd replacement

Did they give you the dvd's?

The startup manager will list all of your bootable partitions then give you a choice of which to boot. Hold down the option key then power on. Continue holding down the option key until you see the startup manager. This brings up the startup manager. Click on your hd or disc. Click on right arrow key.

Restore Tiger 10.4 & Leopard 10.5 DVDs are available from Apple by calling 1(800) 275-2273. Have your serial number ready. Have your credit card ready too. There may be a small fee.

https://discussions.apple.com/thread/4720126?tstart=0 -- January 20,2013

https://discussions.apple.com/thread/5772721?answerId=24588313022#24588313022 -- January 22,2014

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Format a disk using the installation DVD

How to run disk utility from your startup DVD.

1. Insert your startup DVD into your reader. Power down your machine. Hold down to the c key. Power on your machine. This will bootup your startup DVD. ( Alternatively, you may hold down the control key, this brings up the startup manager. Click on the desired volume. Click on the right arrow. )
2. This will bring you to a panel asking you for your language. Pick your language.
   
   
3. You you come to the Install Mac OS panel. Do not install just yet.
4. Click on Utilities menu item. This will give you a pulldown list of utilities.
   
   
5. Click on the disk utility. This will start up disk utility.
   On the left pane view, you will see a list of all your disks. Click on the external disk.
   Click on the partition tab.
   

   

   
   
6. You will now see how your external disk is currently set up. Fill in the information as appropriate. You should pick Mac OS Extended (Journaled).
7. You may wish to verify the correct partition type. Disk Utility should pick the correct partition type for your machine. Click on the "Option" button near the bottom of the window. Select "Apple Partition Map" for the PowerPC, G3, G4, and G5. PowerPC Macs require the Apple Partition Map for their boot drives.
   Note: GUID partition type is for booting Intel Macs. Intel Macs are from 2006 to the present. Master Boot Record is for disk or flash drives you want to share with Windows. Swampus pointed this out to me.

Additional information

More details on formatting.
http://www.kenstone.net/fcp_homepage/partitioning_tiger.html

To format the startup drive, you will need to run disk utility from your installation DVD.

This article will tell you how to get to disk utility. Once in a disk utility, you can go and format the disk.

http://support.apple.com/kb/TS1417

To format your startup drive, you will need to run disk utility from your startup DVD.
Mac OS X 10.4: About the utilities available on the Mac OS X 10.4 Install DVD

http://support.apple.com/kb/HT2055

Install Linux on the box.

I have heard of apple giving out the dvds for free. I've heard there is a twenty dollar fee.

Robert