Indeed it is, Windows dynamically adjusts core usage across the cores according to demand so don't worry about it...
Your task manager shows 8 graphs, and your specs indicate you have quad core i7 2600k which is hyperthreaded so that 1 core is assigned to 2 logical cores and can work on two threads simultaneously, thus appearing to the system as 8 cores...
Hope that helps..
EDIT: If you want to see all 8 cores in action, try using this software, It'll force all eight logical cores to work
Attachment 219592
Unzip it, run LinX.exe with your task manager open, and start it up... watch the graphs then...