I have retextured townguardarmor and it works right with nifskope, but when I tested it in game only half of it was visible -> the whole upper body became invisible. If anyone knows how to solve this problem I'll be grateful for that.

Make sure you have the normal map created, with the same name as the texture (with the _n on the end of the name)
Also, make sure you made the textures with at least 4 mip maps.

Thanks! The problem solved when I added that "_n" on the end of name. Another problem I have is about the icons, the warning post comes and says there are too many mip maps. Truth is that I don't know what mip map means so I have no idea what to do. (I never had this kind of problems when I modded Morrowind.)

Mip Maps is an option when saving the .dds file for icons you want to leave the generate mip maps option unchecked ! --- the Mip Maps are power of 2 steps down in resolution that are saved along with the texture and the game uses 4 levels of them for the various resolutions of items as you approach or walk away from things it reduces the mip map size instead of keeping it in max detail to save n video memory usage - but since icons are always the same size (in the inventory window) they do not need any mip maps generated. - so the CS will warn you if it finds them when you assign an icon - though it usually does not create any problems other than wasting some filespace it can lead to blurred out icons at some resolutions\texture sizes (for example the devs messed up with the KOTN icons and users playing with medium or small texture size will have messed up icons for those items !)

Thanks for help. I have continued my mod for quite a lot, but now I have a problem with scripting. I can't get idea how to make a script which will unlock four doors when a creature is killed. I think it's need to be something like that:

Scriptname KOCdeathofminotaur
begin OnDeath
Unlock (what I'm supposed to have here?!)
end