Their biggest problem is that Apple out-did them in their own game. Their OS is both more secure and more user friendly than Windows.
Hardly! Apple is about "fashion", an Apple user is more concern if the Apple's logo is well sticked to the computer rather than if it's a double-priced PC. Also their OS is far from impressive either on usability or software availability.
But hey! Back when Windows won the World it was the other way around; Windows was damn unsafe and user unfriendly than MacOS...
Linux is a big threat to them in the server space but not in desktop.
Linux keeps doing wrong turns, the only one coming close to threat Windows, Lindows, was sued by Microsoft... they got it covered. I wonder if the new look of Ubuntu wasn't input by MS hired spies to grant that thing sinks before become any threat to Windows.