Not sure it makes good sense, but you can use mouse events, ie.:

Code:
Private Sub txt1_MouseDown(ByVal Button As Integer, ByVal Shift As Integer, ByVal X As Single, ByVal Y As Single)
    txt1.SelStart = 0
    txt1.SelLength = Len(txt1.Text)
End Sub
An obvious issue with this approach is that you couldn't click somewhere else in the text box without selecting all of its text again. You could do the selecting from the right click instead of the left, if that would help at all.