1. Insert the Mac OS X Install disc, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu at the top of the screen. (In Mac OS X 10.4 or later, you must select your language first.)
*Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.*
3. Click the Erase tab.
4. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
5. Select your Mac OS X volume.
6. Highlight the drive, select Partition Tab, then Format type... MacOS Extended Journalled, select the Security Options button, choose Zero Out Data, Erase... after completion hopefully you'll be able to get a good install.
That second Disc thing happens frequently, the best way around it is to do a Custom install, and eliminate enough Printer Drivers, Languages, Fonts, and Applications you don't need... then it may skip #2 altogether. Reboot & hold the Option/at key as well as the left Mouse button to eject Disc#2 & bring up the Startup manger to choose Disc #1 again.