os.utime() uses the following code when times is None under Windows:

        SYSTEMTIME now;
        GetSystemTime(&now);
        if (!SystemTimeToFileTime(&now, &mtime) ||
            !SystemTimeToFileTime(&now, &atime)) {
            PyErr_SetFromWindowsErr(0);
            goto exit;
        }

The problem is GetSystemTime has poor resolution (milliseconds). Instead, it could call GetSystemTimeAsFileTime which writes directly into a FILETIME structure, and potentially (?) has better resolution.

(according to a comment on MSDN, "Resolution on Windows 7 seems to be sub-millisecond": http://msdn.microsoft.com/en-us/library/windows/desktop/ms724397%28v=vs.85%29.aspx )