cpu is 100% used, yet no application shows significant usage
Thanks to some helpful answers I managed to find the answers to this question.

The main cause of this CPU usage is Norton AntiVirus.
A Setting in the AntiVirus called 'Norton Deepsight Community Submission' makes it constantly use around 50% CPU, just to collect data and send this to Norton I guess.

The other 50% is also caused by Norton AntiVirus, yet I am not a 100% sure which setting or process causes this.

Hopefully this helps other people in the future.

Many thanks to the commenters on my main post, which helped me find the cause of this problem.