Before any of these procedures is attempted, at least two backups are important to preserve your data. And to be even safer, shut down and remove the backup media from your machine before continuing, as some media get upset when being connected during an update process.
"Clean install" is not any Mac OS X's installation option. Snow Leopard adds a new twist, in that there isn't an Archive and Install per se. For Mac OS X 10.2 through 10.5 there is an Archive and Install. If you hear people say "Clean Install" about Mac OS X, please correct them as that is not the term Apple uses for Mac OS X installation.
Clean install is what you do to Mac OS 8 and 9 (8.0, 8.1, 8.5, 8.6, 9.0 - 9.2.1) when you create a fresh System Folder, and rename the old one Previous System Folder and have to manually move Preferences, control panels, and extensions over to the new system folder from your old application installations. These operating systems and how to get their updates is described here:
Mac OS 8 and 9 are not to be confused for Mac OS X 10.8 and 10.9, or iOS 8 whose names sound similar.
Mac OS 9 was last bootable on 2003 Macs, and last current in 2001. Mac OS 8 was originally made in 1997.
Mac OS X 10.8 didn't come out until 2012. And 10.9 in 2013. iOS 8 came out in 2014, and is only for iPhone, iPod Touch, and iPads.
In Mac OS X, you have four installation options, one of which is like a Clean Install but goes under a different name. There is:
1. Erase and install - erases the hard drive and installs a fresh copy of the operating system. Currently not recommended because
of a bug in 10.3 that has caused some Firewire hard drives to become unreadable after installation of 10.3. Unless of course you have two copies external of your internal hard drive of every critical file and know how to access them easily, erase and install will only leave you with the backup of your data, assuming the backup is accessible after the installation of the system. With 10.3, there are no guarantees this will be the case with external Firewire hard drives. The problem has been narrowed down to Oxford 922 and 911 drives and Apple is working to solve this problem.
Note, on Macs that can boot into Mac OS 9, be sure to install Mac OS 9 drivers if wish to continue to be able to boot into Mac OS 9 when done with this type of install.
For a complete unrecoverable zeroing of the hard drive see this article http://support.apple.com/kb/HT1820
2. Restore system - restores the operating system and any applications that came with the Mac. You may erase and restore Lion with this tip if your Mac came with Lion, or already has Lion installed.
3. Archive and Install - first appeared under retail 10.2 CDs, is available under both 10.3 update CDs (the $20 variety) and 10.3 retail CDs. It was an option that stopped with Mac OS X 10.6. This gives you the option to create a new system folder, while renaming the old, and save user and network preferences. If you don't save user and network preferences, your installed applications that are non-Apple, and your Users folder get moved to the Previous System folder along with the previous operating system. This is the one most like the Mac OS 8 and 9 Clean Install option, but it is not called that by Apple. This is the currently recommended way to install a new operating system to make sure you preserve your data. Archive and Install may leave behind newer applications than will run with the operating system if you Archive and Install 10.2 over 10.3. It is recommended you move Apple programs replaced by Archive and Install out of the Applications folder before you Archive and Install 10.2 over 10.3.
4. Upgrade Install - Available on upgrade disks of all versions, as well as retail disks. This will simply upgrade an existing X system to a new one, not concerning itself if any old system preferences are incompatible with the new system. This is usually not the recommended option because of its lack of sensitivity to the possibility an old system may not have compatible parts. It is however, also the installation which happens if you select no installation option. As such, it is recommended you only do this if you have no non-Apple applications installed, and all your Apple applications are up to date.
An updated version of this FAQ can be found here*: http://www.macmaps.com/cleaninstall.html
+- * Links to my pages may give me compensation.+