As a last resort and only IF you know what you're doing, and the uninstall feature definitely isn't working... you can simply delete the folder containing your old NWN game and then remove it from your add/remove programs list via regedit.
If you don't know how to do that (or don't want to wreck your computer if you make a mistake while following instructions on the internet) take it in to a computer shop and they should be able to do it in 5min for you.