What is AppleMobileDevice - System user - in Task manager?
AppleMobileDevice is a service that gets installed along with iTunes on Windows.  You need this if you're going to use any Apple devices with your iTunes software.  One example is managing your iPhone, iPad, or iPod.  It also allows for the use of tethering internet from your iPhone/iPad to your PC if your device/provider allows it and the personal hotspot feature has been turned on.

As for why it's taking up so much CPU, that sounds like it could be a problem, for which the answer provided by bneely may have a solution.  If you don't have any Apple devices connected, you could just kill that process and it won't affect anything else.  In order to use your Apple devices normally later on, you would need to re-launch that service somehow, or restart the PC.