I saw this this short film clip on the net that was supposed to be a
meme from MTV.

It showed a boy on Christmas Day unwrapping his present, a lightsaber.
So he switches it on, waves it around the room and slices his
grandma's head off.

Well, I thought it was funny, maybe you had to be there.

Where can I find this film clip?

Hey Markabe,

Gotta love the hysterical things that stick in our heads that we see
on a daily basis. I personally have always had a memory stick for the
great googly moogly commercials myself.
It didn't take that long to find the clip you were referring to.

I did a search on google.com using the keywords: mtv lightsaber clip

So here it is, available for download or streaming video. 

http://www.mtv.co.uk/mtv.co.uk/en/dynamo/features.jhtml?id=30013584

Enjoy, and thanks for the good laugh,
Mother911-ga

That's the one! - ahhh, funny violence, you gotta love it!