Try and start boot2docker with a minimal PATH.
As I illustrated in "How to get Docker to run on a Windows System behind a corporate firewall?", I use:
Path=C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0;C:\prgs\git\latest\bin;C:\prgs\git\latest\usr\bin;C:\Program Files\Oracle\VirtualBox;c:\prgs\Boot2DockerforWindows;C:\Users\43640119\prog\b2d\
That is
C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;
C:\Windows\System32\WindowsPowerShell\v1.0;
C:\prgs\git\latest\bin;
C:\prgs\git\latest\usr\bin;
C:\Program Files\Oracle\VirtualBox;
c:\prgs\Boot2DockerforWindows;C:\Users\43640119\prog\b2d\
With C:\prgs\git\latest
being a symlink to C:\path\to\PortableGit-2.4.5.1-4th-release-candidate-64-bit\
: Git-for-Windows includes the latest from Git, with a recent msys2-based bash.
And VirtualBox being 4.x, not the last 5.0, as mentioned by hzpz