Crossposting here:
I finally got my NWN Diamond to work perfectly! Followed the instructions
here (and on the thread linked in the first reply), then once the game was installed, I right-clicked the "nwn.exe" and did the Windows 7 Compatibility Troubleshooter (lets you run as administrator, fools it into thinking you're under Windows XP etc.) Now the game is smooth like butter and my Sorceress is taking to the plague-ridden streets once more.