Ok, thanks guys.
So it seems that there is not a huge risk if you don't have a ground wire? (as long as you prevent any electric components from touching the case.)
It works in a different way too. You are the source of static electricity. When the case is properly grounded, the static charge bypasses the computer components and is safely discharged through the grounding. Within the case, as many components as possible can be and should be touching, but the ground should be only at one point. More info here and elsewhere: http://siber-sonic.com/electronics/GLoopwhatis.html