Are you talking about "Type Ahead Ability" or are you talking about running a query against records based upon input?

Type Ahead Ability
Code:
Option Explicit

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

Private Const LB_FINDSTRING = &H18F 'for listboxes
Private Const CB_FINDSTRING = &H14C 'for comboboxes

Private Sub Form_Load()
Combo1.AddItem "this is a test", 0
Combo1.AddItem "so is this", 1
Combo1.AddItem "but not this", 2
Combo1.AddItem "Well really it is", 3
Combo1.AddItem "in fact", 4
Combo1.AddItem "all of this", 5
Combo1.AddItem "is a test", 6
End Sub

Private Sub Combo1_KeyUp(KeyCode As Integer, Shift As Integer)

Dim Idx As Long, TempString As String

If KeyCode < 32 Or KeyCode > 126 Then Exit Sub

If KeyCode = vbKeyBack Then Exit Sub

TempString = Combo1.Text

Idx = SendMessage(Combo1.hwnd, CB_FINDSTRING, -1, ByVal TempString)

If Idx <> -1 Then
  Combo1.ListIndex = Idx
  Combo1.SelStart = Len(TempString)
  Combo1.SelLength = Len(Combo1.Text) - Len(TempString)
Else
  Combo1.Text = TempString
  Combo1.SelStart = Len(TempString)
End If

End Sub
Good Luck