Thanks for your help, I'll try that. I was inside Thieves guild when it happened the first time and I don't think it's related. It happens every time I reload any saved game, so I thought I have to delete all of them and start again, but I'll try deleting the dynamic files first, I hope it works!
Oh, and it doesn't matter if go around and kill some monsters, save and then load, whenever I speak to a NPC after that the game freezes.
