Sorry I missed this - I can reproduce this on Windows 8.1, but not on Windows 7. I hope I'll be able to try another Windows 8.1 machine today.

SYSTEMROOT is definitely set in the original environment:

    >>> os.environ['SYSTEMROOT']
    'C:\\Windows'
    >>> subprocess.call('{} /c set SYSTEMROOT'.format(cmd32), env=os.environ)
    Environment variable SYSTEMROOT not defined
    1
    >>> subprocess.call('{} /c set SYSTEMROOT'.format(cmd32))
    SystemRoot=C:\Windows
    0

It seems only a minimal set of environment variables are set in the spawned process:

    >>> subprocess.call('{} /c set'.format(cmd32), env=os.environ)
    COMSPEC=C:\Windows\SysWOW64\cmd.exe
    PATHEXT=.COM;.EXE;.BAT;.CMD;.VBS;.JS;.WS;.MSC
    PROMPT=$P$G