you can do something like this:

Private m_iScrollDir As Integer 'Which way to scroll
Private mfX As Single
Private mfY As Single
Private mfInDrag As Boolean
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" _
(ByVal hWnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As _
Any) As Long

'mfInDrag set true in drag start, false in drag end

'TV event
Private Sub moTV_OLEDragOver(Data As MSComctlLib.DataObject, Effect As Long,
Button As Integer, Shift As Integer, x As Single, y As Single, State As
Integer)
   mfX = x
   mfY = y
'Scroll treeview
   If y > 0 And y < 400 Then 'scroll up
       ' Send a WM_VSCROLL message 0 is up and 1 is down
       m_iScrollDir = -1
   ElseIf y > (mctlTV.Height - 500) And y < mctlTV.Height Then
       'scroll down
       m_iScrollDir = 1
   Else
       m_iScrollDir = 0
   End If
End Sub

'Form's event
Private Sub mfrm_Timer()
   Static lngStayCount As Long, strStayNode As String
   If mfInDrag Then
       Set moTV.DropHighlight = moTV.HitTest(mfX, mfY)
       If m_iScrollDir = -1 Then 'Scroll Up
       ' Send a WM_VSCROLL message 0 is up and 1 is down
         SendMessage moTV.hWnd, 277&, 0&, vbNull
       ElseIf m_iScrollDir = 1 Then  'Scroll Down
         SendMessage moTV.hWnd, 277&, 1&, vbNull
       End If
   End If
End Sub