Sometimes when I try to hack a computer in the game it shows some strange fallout icons for a second, http://media.snimka.bg/7495/021244879.jpg (in PC version)
Does anybody know, is it some secret game option or it is just a bug?

This is a glitch. A very creepy one. :shrug:

I assume that the image file that contains the icons you see has been assigned an incorrect name and they appear in places they shouldn't. If you are using any mods, it's almost certain that one of them causes this.

EDIT: For your information, press PrintScreen for a second or two to take screenshots.