Be sure to backup all the files you want to save before doing any of the following since after you follow the steps you will
NOT be able to recover any data. You can burn important information to a CD or a DVD or you can also backup data to another Hard Drive or computer.
How to erase the Hard Dirve and zero all data
(These steps assume you have a Mac OS X 10.2.3 or later CD)
1. Insert the Mac OS X CD.
2. Restart the computer.
3. Immediately after the startup sound, press and hold the "C" key to start up from CD.
4. When the Installer screen appears, do not click Continue. Instead, choose Installer > Open Disk Utilities.
5. Select the hard drive to erase.
6. Click the Erase tab.
7. Select the volume format from the Volume Format pop-up menu.
8. Click Options.
9. Select the checkbox for "Zero all data".
10. Click OK.
11. Click Erase. (This may take a long time depending on the size of your hard drive)
More information can be found here:
http://docs.info.apple.com/article.html?artnum=107437 which is where I copied the information from.
Good luck
