For a system-agnostic solution that works on both Windows and Linux, no matter what the input path, one could use os.path.join(os.sep, rootdir + os.sep, targetdir)
On WIndows:
>>> os.path.join(os.sep, "C:" + os.sep, "Windows")
'C:\\Windows'
On Linux:
>>> os.path.join(os.sep, "usr" + os.sep, "lib")
'/usr/lib'