First: don't worry about the fact that NWN is listed as being only a few MB big in the software list. That's nothing unusual.
Windows has a rather... *chuckle*...
creative... way of calculating that value, which isn't really bound to the actual size of the program folder. To give you a comparison: in my case it says something about "1,09MB" while my actual fodler is several Gigs large. Nothing special. Praised be Gates.
Second, did you already patch it to the latest version? That's always worth a try. If that doesnt work... hmmm, something probably screwed up your registry entries. I'd try reinstalling the whole thing a last time, but this time in a different folder (in a weak attempt to avoid the old registry settings) and to install HotU
before installing SoU... if that is possible, since actually I never tried that. =/
Can't think of much else right now...
-Zel