If you have a newer ATI GFX-Card like me (one which doesn't use legacy drivers like Catalyst 9.12) there is only one 100% working way to watch the movies ingame (which I know of):
Edit your nwn.ini:
[Display Options]
FullScreen=1
AllowWindowedMode=1
UseLargeFont=1
SafeMovie=1
Disable Movies=0
Disable Intro Movies=1
Then change the resolution to one which is lower than your native/desktop resolution ingame, when a video gets triggered press: alt+enter to switch to windowed mode. Might be safer to be in windowed mode before the video gets played (chapter intro or chapter change and before endgame)
Voila! You should see the video again.
There are sites which offer other possible solutions but none worked for me:
http://pcgamingwiki.com/wiki/Neverwinter_Nights If someone knows a solution which works in fullscreen mode and with windows I'd like to hear about it ^^
(nwmovies on linux can play the vids just fine provided you use a desktop without direct rendering)