I killed Boratus, and now I want to continue the quest (going to the Dwarven Halls for the priest), but i don't get that part of the quest, because I can't open the box in Boratus' room. I seems that the key disappeared...

He should have dropped the key. did you let the wolves have a go at him first?
Kiya

Run your mouse around the wall, etc. where he died; the key may just be hidden (too bad Alt descriptions do not show through obstructions). If not, do you have a recent save you could re-load and kill him again?

As a last resort, check out (on the old forum);
Lost Items - guide and directory of inventory files (inv.*)