Hi I was making a custom bow but how do I do to rig the string? movement animation etc?

http://skyrim.nexusmods.com/mods/13604 And he's made several bows.

Ok but I see there are three types of models nifs like ...

bow skinned
bow skinned hkp and no hkp what's that?

I'm sorry, I don't know where you're seeing hkp files. Are they animations? I don't think you'd want to change them. Bowskinned is the weapon itself.

They are other bow files with different bones inside ...