How to select the contents of a textbox once it is activated?

Can't be more simple than this I guess...

Private Sub TextBox1_MouseDown(ByVal Button As Integer, ByVal Shift As Integer, _
ByVal X As Single, ByVal Y As Single)
    With TextBox1
        .SelStart = 0
        .SelLength = Len(.Text)
    End With
End Sub

Whether you click on the textbox or you tab into it, it will do what you want. To deselect the text, use the arrow keys.

Explanation

If you debug the code you will see that even though you have said .SetFocus, the focus is not on the Textbox. .SetFocus doesn't work in TextBox1_Enter() and you need to have focus for the rest of the code to work. And hence my alternative...

Alternative

You may also like this version :) This overcomes the limitation of using the mouse in the TextBox

Dim boolEnter As Boolean

Private Sub TextBox1_Enter()
    boolEnter = True
End Sub

Private Sub TextBox1_MouseDown(ByVal Button As Integer, ByVal Shift As Integer, _
ByVal X As Single, ByVal Y As Single)
    If boolEnter = True Then
        With TextBox1
            .SelStart = 0
            .SelLength = Len(.Text)
        End With
        boolEnter = False
    End If
End Sub

Pff, took me a while. Actually, your code works, but it highlights the text BEFORE the click event happens. So you clicking in the box instantly overrides the selection created by the code. I have used a delayed selection, and it works, though it is a bit disgusting...

The code for the textboxes:

Private Sub TextBox1_Enter()
  Application.OnTime Now + TimeValue("00:00:01"), "module1.SelectText1"
End Sub

Private Sub TextBox2_Enter()
  Application.OnTime Now, "module1.SelectText2"
End Sub

Note that it works even withouth the {+ TimeValue("00:00:01")} part, but it might theoretically stop it from working at times. Hmm, on a second thought, just leave it out. I doubt it would ever cause a problem.

Now the code in module1:

Sub SelectText1()
  UserForm1.TextBox1.SelStart = 0
  UserForm1.TextBox1.SelLength = Len(UserForm1.TextBox1.Text)
End Sub

Sub SelectText2()
  UserForm1.TextBox2.SelStart = 0
  UserForm1.TextBox2.SelLength = Len(UserForm1.TextBox2.Text)
End Sub

Hope this works for you too. Ineresting problem. :) Cheers!