To start up from an optical disc or a flash drive, insert it, then restart the computer and hold down the C key at the startup chime. A retail disc, or the gray discs from another model, will not work. You should have received those media from the original owner, but if you didn't, order replacements from Apple. If the machine shipped with OS X 10.6, you need the installation media that came with it: gray installation discs, or a USB flash drive for a MacBook Air. Preferably, install as much memory as it can take, according to the technical specifications.

If the machine is very old and has less than 1 GB of memory, you'll need to add more in order to install 10.6. If the machine shipped with OS X 10.4 or 10.5, you need a boxed and shrink-wrapped retail Snow Leopard (OS X 10.6) installation disc from the Apple Store or a reputable reseller- not from eBay or anything of the kind. It's unsafe, and may be unlawful, to use a computer with software installed by a previous owner. Then find the model on this page to see what OS version was originally installed. If you're not sure of the model, enter the serial number on this page.

How you go about it depends on the model, and on whether you already own another Mac.

Changes made by Apple over the years have made this seemingly straightforward task very complex. The first thing to do with a second-hand computer is to erase the internal drive and install a clean copy of OS X.

Instal the new for mac Old Snook