I managed to fix my problem by taking out the texture pack I realized this as I set up tekkit for my brother and it worked till I put on sphax with the tekkit patch. Click start and type %appdata% and click technic. Go into texture packs and delete it
I have a similar problem but its when I click on multiplayer, single player, options, etc. that it closes. It used to happen with voltz as well but now its only tekkit. I'm not sure if these problems are linked but I haven't been able to get on to my tekkit world which was at the flying rings and quantum armour stage.