Reinstalling "in place" will replace your Mac OS and system items. It should "spare" third party drivers, applications, etc. This is very useful for problems with Mac HD's that are not booting properly (or at all). This is recommended only after using disk maintenance tools, like Norton Utilities, Disk First Aid, etc.

1. If the computer is running, place the "Power Mac G4/G3 INSTALL" (NOT the Restore CD) CD in the CD/DVD drive and wait for it to mount. Then choose "Restart" from the Special menu. -Immediately- press and hold the "C" key.
If the computer is not running (or will not boot). Startup the Mac, immediately open the CD/DVD drive, and insert the "Power Mac G4/G3 INSTALL" CD. Then press and hold the "C" key until the computer boots.
Allow the Mac to boot from the Install CD. You'll know that the Install CD is your startup disk, because a) the destkop is a CD pattern, and b) the Install CD appears above all other hard drives.

2. Once the computer boots, the Install CD's directory window will probably open. Double click the icon "Mac OS Install".

3. Follow the steps to reinstall the Mac OS. When it asks you whether to "Reinstall" or "add/remove feature". Choose "Reinstall". Allow the installer to proceed through reinstallation, and restart the computer when prompted to do so.

4. On restart, eject the Mac OS Install CD. If all goes well, the Mac HD should boot normally.