The way you go about a Mac OS X reinstall depends on which version of Mac OS X you have. If you have 10.3 or earlier, I really can't be of much help, as I don't remember how those Mac OS X versions worked. If you have 10.4 (Tiger) or 10.5 (Leopard), do an Archive and Install, preserving users and network settings. If you have 10.6 (Snow Leopard), the installer will do this for you.