I had this same problem when I first played NwN on Vista. It drove me crazy and I was never able to find a fix for it. I finally set up my system to be able to dual-boot into Vista or XP and used XP exclusively for NwN. Since then I've upgraded to Windows 7 which plays well with NwN. If your OS is Vista then I think that might be the problem.
My Main Toons: (80) [9x] Mary Jane (Druid) (78) [7x] Brutus (Barbarian) (77) [7x] Damien (Cleric) (77) [7x] Requiem (Bard) (76) [5x] Ein Stein (Pariah) (71) [4x] .Sirius. (Threader) (68) [2x] Jinn Li (Monk)