Origin

Switch was an advertising campaign launched by Apple Computer (now Apple Inc.) on June 10, 2002. It featured what the company referred to as "real people" who had "switched" from the Microsoft Windows platform to the Mac. An international television and print ad campaign directed users to a website where various myths about the Mac platform were dispelled.

(Source, wikipedia, see link)

Internet phenomenon

One of the ads' goal was to point at Microsoft's computers shortcomings. While the effectiveness of that campaign didn't get very far, the simplicity in the making (an actor and a white background) led to image and video parodies the same year:

(The embedding link seeming to have some problems, here is the URL for the video:http://www.spike.com/video/mac-switch-parody/2463684)

Most of the parodies lamented features they had lost due to the switch, such as the ability to play popular games and use of the right mouse button.
You can find here some more of it.

Even Microsoft was tempted to follow that trend, as this article shows. While they did make a website dedicated to the staged testimony of someone switching a Mac to a PC, they apologized and took it down afterward.

On YTMND

By 2005, that trend reached YTMND, leading to 6 creations mostly done by the same user of different well-known characters reenacting the ad.
The most popular (with more than 21,000 views) was the Severus Snape one:

• http://switch.ytmnd.com/

On Youtube

In 2005 too, Youtube began to be a streaming platform featuring a great amount of parodies from the ads:

You can find way more video parodies here and here