Did you get this error after reinstalling the game or before?
The message sounds like the game is already installed but the launcher cannot find it. Perhaps there is an autorun file on the DVD that tries to launch the game when the DVD is inserted and a registry entry (from an old install) is still there, yet the actual directory isn't.
If you haven't already reinstalled the game, go to your Windows Start menu, choose Computer, right click on the DVD drive and choose open. There should be a file called setup.exe, install.exe or something like that. Run it to reinstall the game.
If you've already done so, find the install directory on your computer (probably in documents and settings → application data → nwn2 or something) and look for an ini file that's called install.ini, launch.ini, nwn2.ini etc. (any .ini, actually, since I have no idea what the game calls it). Open it with a text editor and (right click, open with... notepad) search for a preset install dir and modify the path according to the path you installed nwn2 to.
Or wait for Catch-Up and her husband('s friend).
Edit: here's the same problem for NWN1, perhaps it helps:http://social.bioware.com/forum/1/topic ... /3341490/2