cpu is 100% used, yet no application shows significant usage Since a few days my IMac started showing weird behaviour.
In Activity Monitor it shows that my CPU is 100% used. Literally 0% idle, but there are no Applications using anywhere near this much resources.
The highest percentage would be from Activity Monitor itself, but this just uses 2.5% at max.
The amount of CPU used is shared over 'User' and 'System' they both use around 50% CPU at the same time.

Also, the amount of threads running is around 300 to 375, but the sum of all running applications their threads doesn't go over 100.

My question is:
Is there any explanation for this behaviour?
And can I do something against this? 

Info about my Mac:

OS : Mac OS X 10.7.5

CPU : 3.06 GHz Intel Core 2 Duo

Memory : 4 GB 1067 MHz RAM

(iMac Late 2009)

EDIT:

After making sure that I was looking at 'All Processes' that are running on my iMac, I found that Norton Deepsight was causing a big part of my issue.
By changing a setting in my Norton-AntiVirus I managed to take atleast 50% of CPU usage away.
Yet my System is still using around 50% to 60% of CPU, which is caused by 'kernel_task' and 'SymDaemon'

Is there a clear reason to why these to processes are using this much CPU?
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.