I am using Linux Slackware with kernel 3.6.0 and KDE 4.5.5. Most of the time I am developing web scripts and have 2 applications open: Emacs and Firefox. When I am typing something I hear the HDD constantly clicking and fans cooling the CPU. But I suppose the machine to almost idle when I am typing in Emacs.

I looked into the system monitor, it says, that X, ksysguard and firefox are doing something in the background. What can it be? How to stop this activity?

Cause it's really pleasant to work on a silent machine and not to hear the constant clicks.

I am sure, that I don't have any viruses/trojans.

X provides the GUI. KSysGuard is a KDE system monitoring tool; I don't use KDE, but perhaps it can be removed from the list of startup applications. Based on this article, it doesn't appear to be an essential KDE process.

Neither should put a significant load on your system, unless your system is seriously underpowered; you can run top in a terminal to see what processes are eating what resources.

Based on your statement about the fans, my first guess is that your system may not be cooling properly. Make sure the vents are clear of dust and that the fans are blowing air. If it's a desktop, open the case and make sure the CPU and CPU fans are clear of dust. Dust is an enemy of computers; it also conducts electricity and, in extreme cases, can short circuits. A can of compressed air is recommended, as an electric vacuum cleaner might cause static.

You might also use a monitoring tool, such as KSysGuard, GKrellM, or Conky, to monitor the CPU temperature.

Ok, it was firefox. The solution can be found here:

http://www.wikihow.com/Stop-Firefox-...our-Cpu-Cycles