CListView Vista Problem

We are modifying a CListView Control as follows:

void CMyListView::OnInitialUpdate()
{
  CListView::OnInitialUpdate();
  
  ...

  DWORD dwAdd = LVS_REPORT | LVS_SINGLESEL | LVS_SHOWSELALWAYS |
LVS_OWNERDATA;
  DWORD dwRemove = LVS_TYPEMASK;

  GetListCtrl().ModifyStyle(dwRemove, dwAdd);

  ListView_SetExtendedListViewStyle(GetListCtrl().m_hWnd,
LVS_EX_FULLROWSELECT |
    LVS_EX_ONECLICKACTIVATE | LVS_EX_GRIDLINES |
    LVS_EX_HEADERDRAGDROP | LVS_EX_CHECKBOXES);

  CHeaderCtrl* pHeaderCtrl = GetListCtrl().GetHeaderCtrl();
  ASSERT(pHeaderCtrl != NULL);

  if (pHeaderCtrl != NULL)
  {
    DWORD dwAdd = HDS_HOTTRACK | HDS_BUTTONS | HDS_DRAGDROP | HDS_FULLDRAG;
    DWORD dwRemove = 0;
    
    pHeaderCtrl->ModifyStyle(dwRemove, dwAdd);
  }

  ...
}

Our code handling the click of the CMyListView contains:

void CMyListView::OnClick(NMHDR* pNMHDR, LRESULT* pResult)
{
  int iItem;

  NM_LISTVIEW* pNMListView = (NM_LISTVIEW*)pNMHDR;

  if (!pNMListView->uOldState && !pNMListView->uNewState)
    return;

  // Process the click
  ...
}

When we use this control with the above custom code in Windows XP, Clicking
in the checkbox results in:

pNMListView->uOldState = 1031
pNMListView->uNewState = 0

and the rest of our code executes just fine.

When we use this control in Windows Vista, Clicking in the checkbox results
in:

pNMListView->uOldState = 0
pNMListView->uNewState = 0

and the rest of our code does not execute.

If I remove the check "if (!pNMListView->uOldState &&
!pNMListView->uNewState)", the code executes just fine, but I'd like to
understand why the values are different for XP and Vista before removing this
check.