It is possible for a Windows user to sit down at a Linux computer and use it. However, the one of the most exciting things about Linux (in my opinion) is the ability to know and control what is going on "under the hood". You should teach your student to go beyond the GUI, and into the file system.
Graphical Desktop
Although you are going to go much deeper into the workings of the operating system, you should start your student off in an environment that they are familiar with. I would recommend starting with KDE, since it's more Windows-like than GNOME. By that, I mean the "Programs" menu is on the bottom left, the task bar is on the bottom of the screen, and the menu system has a similar feel.
Also, it is a good idea to set them up with programs that are similar, if not the same, as their Windows counterparts. Start off with Open Office, Firefox and Evolution for word processing, web browsing and email, respectively.
File system
Once your student is comfortable with the desktop, it's time to delve into the file system. The Linux file system is comprised of multiple directories in a single directory tree, with root (/) being the base. Each directory beneath the root directory has a specific function. Tolga BALCI's article has a good overview of what each directory does.
It is important for your new Linux student to understand that all files, directories, and hardware devices are seen as a file by the kernel. This will assist in the understanding of why the file system separates directories the way it does. It will also assist in the understanding of how the kernel interacts with physical devices.
While you are discussing the file system, you will also want to discuss the difference between the administrator or root user and a regular user. Although it has become rather commonplace to replace the root user with the sudo command, there are still processes and files that are owned by root. This is an important security feature in Linux, and unlike Windows, the user will be prompted for a password when making changes to any system file or system-wide process.
Basic Commands
Next, you should discuss the common Linux commands and their meanings. Graphical interfaces are becoming more and more common for regular system maintenance and use, but there are still some things that require command line actions. You will not be able to teach them every Linux command, but having an idea of how to move about the file system, remove files, extract information about their system, and execute commands as root is a good place to start.
Where To Go From Here
That is a basic overview of how to teach Linux to Windows users. Of course, there is a lot more information that should be passed on. Some information is distro specific, like how to add and remove programs. Other information depends on how far the user wants to take it, such as shell scripting. However, the above information will give them a better understanding of the differences between Windows and Linux, as well as the similarities, and hopefully wet their appetite for more information.