[ecfisa]

Hola fedee.

Se me ocurre que podés aprovechar el evento OnChange del Edit:

void __fastcall TForm1::Edit1Change(TObject *Sender)
{
  int i = 0;
  boolean Found = FALSE;
  while(i < ListBox1->Count && !Found) {
    Found = ListBox1->Items->Strings[i].SubString(1,Edit1->Text.Length()) == Edit1->Text;
    i++;
  }
  if(Found)ListBox1->ItemIndex = i-1;
}

Saludos.