Instal Leopard from the disc that came with your computer.
1. Turn your computer on and pop in the disc. You might have to hold down the option key while booting and select the Leopard disc from the ensuing menu.
2. After the OS installer has started there should be a menu item at the top of the screen called "Utilities". Click that and select "Disk Utility" from the submenu.
3. After Disk Utility opens, select your hard drive from the source list on the left. Click on the "Partition" tab. Set the "Volume Scheme" to "One Partition" from the drop down menu. Change the name to "Macintosh HD" or whatever you want. Change the format to "Mac OS Extended (Journaled)". Click on the "Options
" button near the bottom of the window and make sure the "GUID Partition Table" option is selected.
4. Click "Apply" and wait for your hard drive to finish formating. When it's done, quit Disk Utility and follow the on screen instruction to install the OS.