Update 2015 September 4: An incorrect paclet update was pushed recently that caused Mathematica to not hang on startup with "Initializing kernels". The problem and solution are described here: Mathematica 10.0.x freezes in Initializing kernels.
In version 10.1.0 under Windows 7 x64 I have a repeating problem. Every time I start the Mathematica Front End the Welcome to Wolfram Mathematica screen appears, then freezes and eventually is marked (Not Responding) by Windows.
(It does not matter whether Welcome screen or New document is selected as the show-on-startup action; the Front End freezes in either case immediately after that object is displayed.)
How can I fix this?