Prior to reinstallation of the MacOS, the hard drive should be erased. Since there is no way to use the more thorough disk wiping tool used with Windows systems, <a href="http://www.dban.org/download"> DBAN</a>, on an Apple system, the alternative is to bring up the Disk Utility application in Go/Applications/Utilities on the Mac and select the 'Erase' tab. 
Click on the 'Erase Free Space' button and then he 'Erase...' button.  You would need to borrow the specific Mac OS version you want to install from the help desk if you don't already have the DVD.
<p>
Typically, you have the option to reinstall the OS after the disk has been erased.
<p> 
However, Mac OS 10.7 and later do not have this feature and you will need to install the OS from another source.  The result is that you now have a blank disk with no means of reinstalling the OS from a DVD (disk is blank=no drivers)..  One way to get around this is to connect the system to another identical system using a firewire cable. 

<p> Open the Disk Utility app on the system that has the OS you want to copy to the blank disk. In the left column you should see the drives for each of the laptops.  One will be grey and labelled <b>MacIntosh HD</b> and the other will be a light orange color and not have a label.
<p> Note that it is counterintuitive to not immediately select the Restore tab at the top of this pane. If you <u>do</u> select 'Restore' at this juncture and input both the source and Destination disks,  you will get the following error message:  <code><b>This disk cannot be used to start up your computer</code></b>.
<p>
The solution is to not pursue 'Restore' just yet.  First, select the 'First Aid' tab in the same row as 'Restore' 
and select the 'Verify Disk' and then the 'Repair Disk' options.  'Verify Disk' is similar to the fsck (file system consistency check) command in Unix.
<p>After these 2 procedures are run, you should see a message like The volume MacIntosh HD appears to be OK' and/or 'The volume MacIntosh HD has been repaired.'
<p>
<u>Now</u> execute the Restore function and copy the MacOS from the working HD to the erased one via Firewire in a matter of minutes.