Literally just created a new project, imported the NGUI package and went to the example scene. It nearly always happens the first time you open the project. If you run it, stop and then run again, its usually ok. However, if you select some of the sprites and/or labels (you don't have to do anything with them, just select them) it'll go wrong again. If you run it and scroll all the way to the end and then back it usually solves itself, though you may have to do it a few times. It's not just on the PC either, I built it and loaded onto my android pad, same result.
BTW great product