I use Visual Studio 2010 Professional at work and up until a couple of weeks ago I have had no problems. But now whenever I open an instance of VS 2010 it defaults to horizontal split. I never split my windows horizontally, so this is very annoying. It happens consistently, every time on every project.

Here is how VS2010 looked before I closed the window:

I close it and reopen in that project, and:

Arg! The only plugin I use is ReSharper, in case it's relevant.