Mac OS X 10.2 introduces a new way to install: Archive and Install. This kind of installation automatically moves existing system files to a folder named Previous System, then installs Mac OS X again. (See tip 3.) You cannot start up your computer using the Previous System folder, nor can you "re-bless" the Previous System folder. (See tip 1.)
You may also choose to preserve your user and network settings before installing. This option automatically imports existing users, Home directories, and network settings. This also skips the Setup Assistant after installation.