How do Dell and other computer manufacturers pre-install operating systems and all the other software on their machines?
Perhaps I can also shortly explain it here for the ones who are too lazy to read Wikipedia :) As you might imagine, computer manufacturers need to make thousands and thousands of identical copies. Installing all programs one by one manually would not only take ages, it would also include the possibility for human errors. Instead, they first install everything on one computer, and then they simply clone the harddisk of that computer to all other computers by creating a disk image and then by transferring the image simultaneously to a large number of other computers. This is just a very short version of the explanation - Wikipedia can help with the details and much much more information on the topic :)
It is done with hard disk images. Have a look at these Wikipedia articles: http://en.wikipedia.org/wiki/Disk_image#Rapid_deployment_of_clone_systems and http://en.wikipedia.org/wiki/Disk_cloning
It's referred to as OEM System Builder Licensing. Here's a link that you might find interesting: