Anonymous
Patents my friend patents! And also ads for any product aren't free! Companies have to pay advertising fees for their products to show up in movies, tv shows!
P
It's a combination of having something that looks clean\sharp on camera and not wanting to give free endorsements to Microsoft or Apple. Your typical computer will have a lot of detail and graphics on screen that will look far too busy on a camera shot.
Anonymous
Sometimes it's because they are worried about trademarks and intellectual property. By avoiding anything recognizable they are less likely to be sued for violating a copyright or associating a product with evil.
don
Here in the Netherlands most series use real operating systems.
keerok
That's because what they're trying to show doesn't happen in real life. Best example? When they zoom in a frozen frame from a CCTV camera, the image magically becomes clearer and sharper. In reality, it's the opposite.