c_char_p.value doesn't return a bytes object on Windows.
http://docs.python.org/3.1/library/ctypes.html#fundamental-data-types states that c_char_p is either a "bytes object or None" in Python, not str.
----------
test_c_bug.py
----------
import ctypes

test_string = ctypes.c_char_p("This Is a test string, that should be of type bytes")

print (test_string.value)
print ("Typeof test_string {}",type(test_string))
print ("Typeof test_string {}",type(test_string.value))

assert(type(test_string.value) == bytes)

-----------------
Windows Xp 5.1 SP3
Python 3.1.1 (r311:74483, Aug 17 2009, 17:02:12) [MSC v.1500 32 bit (Intel)] on win32
-----------------
C:\>python test_c_bug.py
This Is a test string, that should be of type bytes
Typeof test_string {} <class 'ctypes.c_char_p'>
Typeof test_string {} <class 'str'>
Traceback (most recent call last):
  File "test_c_bug.py", line 9, in <module>
    assert(type(test_string.value) == bytes)
AssertionError

-----------------
Ubuntu 9.10 Karmic 
Python 3.1.1+ (r311:74480, Nov  2 2009, 14:49:22) 
[GCC 4.4.1] on linux2
-----------------
david@Waldorf:~/dev/gtype/gtypes$ python3 test_c_bug.py
b'This Is a test string, that should be of type bytes'
Typeof test_string {} <class 'ctypes.c_char_p'>
Typeof test_string {} <class 'bytes'>

FWIW, this isn't an issue on py3k (on Mac or Windows).

You need to change

test_string = ctypes.c_char_p("This Is a test string, that should be of type bytes")

to

test_string = ctypes.c_char_p(b"This Is a test string, that should be of type bytes")

but this issue itself seems to be already fixed.