I've been struggling with this for a while. 2.16 and now 2.18.11. If my Windows 7 X64 system hangs on "Starting up GUI" it will never come back and eventually ends with a blue screen system crash. It seems the network related and I used to disable the network adapter before starting QGIS with mixed results. It's nuts but here's what works for me: start QGIS with the network cable unplugged. If it hangs plug the network cable back in. When the network comes up QGIS finishes its startup.