def get_windows_version(windows_version: str = 'win7') -> str:
    """
    valid windows versions : 'nt40', 'vista', 'win10', 'win2k', 'win2k3', 'win2k8', 'win31', 'win7', 'win8', 'win81', 'win95', 'win98', 'winxp'

    >>> import unittest
    >>> assert get_windows_version() == 'win7'
    >>> assert get_windows_version('nt40') == 'nt40'
    >>> unittest.TestCase().assertRaises(RuntimeError, get_windows_version, windows_version='invalid')
    """
    valid_windows_versions = ['nt40', 'vista', 'win10', 'win2k', 'win2k3', 'win2k8', 'win31', 'win7', 'win8', 'win81', 'win95', 'win98', 'winxp']
    if not windows_version:
        windows_version = 'win7'
    windows_version = windows_version.lower().strip()
    if windows_version not in valid_windows_versions:
        raise RuntimeError('Invalid windows_version: "{windows_version}"'.format(windows_version=windows_version))
    return windows_version