Some little backstory

Since the creation of Microsoft and Apple in the 1970s and the respective PC and Mac computers that ensued, there always have been differences of opinion in knowing wich operating system was the best, the most successful and the most comfortable to use.
But, there was, in the beginning, no real "rivalry" for Microsoft was more focused on Office computers whereas Mac was more suitable concerning graphic rendition and playful use.
For a very long time, these stereotypes have been part of our culture.
Anyway, for users of each side, as far as computer technology developed itself and began a race to the newest update, arguments pointing at each other shortcoming were unavoidable, as this archive from 1992 shows.

The "Get a Mac" ads

Apple, in 2002, already took the cunning initiative to play on that "rivalry" between the two companies, in a subtle way, with their Apple Switch ad campaign.
Those ads faded out quickly in the maintream media and became the target of a memetic spread concerning parodies and Ellen Feiss, without really focusing on some kind of "war" between the two blocks.

4 years later, Apple struke back with their "Get a Mac" ads campaign, being more straightforward this time:

As one can see, this is a presumptuous and arrogant, yet funny, way to express that Macs are better than PCs.
The stereotypes used (PC seen as a classy businessman while Mac seen as a cool young man), along with the simple way of making the ad reused (a white background) caught on immediately.

On Youtube

Youtubes parodies started appearing the same year the ads did.
In most of the cases, it became "Mac vs PC" videos reinforcing this "war" idea. It was a way to make fun of each side, being PC, Mac or even Linux:

You can find many many more videos there
You can also find more spoofs and parodies at MacSpoofs.com.

In 2008, Microsoft responded in a funny way by making their own version of the ad:

Here is a musical version of the meme, summing up the long-lived argument between the two OS (Warning, some gore scenes):

On YTMND

Starting in 2007, about 13 Mac vs PC themed YTMNDs have been made.
The most popular, gathering more than 250,000 views, is http://mvpc.ytmnd.com/ by user Feedrosie.