Like this

Code:
Dim strWhere As String
Dim lngLen As Long
If Not IsNull(Me.txtSurname) Then
strWhere = strWhere & "([PtSurname] Like ""*" & Me.txtSurname & "*"")"
End If
If Not IsNull(Me.txtFilterSite) Then
  If Len(strWhere) > 0 Then
      strWhere = strWhere & " AND "
  End If 
      strWhere = strWhere & "([Site] Like ""*" & Me.txtFilterSite & "*"")"
End If
If Not IsNull(Me.TextUserName) Then
  If Len(strWhere) > 0 Then
      strWhere = strWhere & " AND "
  End If 
  strWhere = strWhere & "([User] Like ""*" & Me.TextUserName & "*"")"
End If
If Not IsNull(Me.Textdate) Then
  If Len(strWhere) > 0 Then
      strWhere = strWhere & " AND "
  End If 
   strWhere = strWhere & "([TransferDate] Like ""*" & Me.Textdate & "*"")"
End If
Me.Filter = strWhere
Me.FilterOn = True
End Sub