Test failures on windows
The included error message is on 32-bit windows for python 2.6 but it's the same for 64-bit (waiting for other python versions to finish)

this is a blocker for 0.13 rc1

```
======================================================================
FAIL: test_groupby_return_type (pandas.tests.test_groupby.TestGroupBy)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\workspace\pandas-windows-test-py26\pandas\tests\test_groupby.py", line 331, in test_groupby_return_type
    assert_frame_equal(result,expected)
  File "C:\workspace\pandas-windows-test-py26\pandas\util\testing.py", line 475, in assert_frame_equal
    check_less_precise=check_less_precise)
  File "C:\workspace\pandas-windows-test-py26\pandas\util\testing.py", line 423, in assert_series_equal
    assert_attr_equal('dtype', left, right)
  File "C:\workspace\pandas-windows-test-py26\pandas\util\testing.py", line 407, in assert_attr_equal
    assert_equal(left_attr,right_attr,"attr is not equal [{0}]" .format(attr))
  File "C:\workspace\pandas-windows-test-py26\pandas\util\testing.py", line 390, in assert_equal
    assert a == b, "%s: %r != %r" % (msg.format(a,b), a, b)
AssertionError: attr is not equal [dtype]: dtype('int64') != dtype('int32')
```
