I have a lengthy process, call it My Process, started by clicking a
button from a userform.

MyProcess can take a few minutes, and while it's running I want all
excel user interfacing disabled. In excel 97 I simply surrounded the
call to MyProcess in CommandButton_Click(), where CommandButton is the
button on the form that starts the process, with lines like the
following setting the userform Enabled property to False and back to
True again:

Private Sub CommandButton_Click()
Me.Enabled = False
MyProcess
Me.Enabled = True
End Sub

This works fine in excel 97, but in excel 2000 and 2003 excel remains
locked up after exiting CommandButton_CLick and I have to do
<ctrl<Break to regain control of excel again. In order to be
convinced that MyProcess is not interacting in some unexpected way
with excel, it is easy to replicate the situation by creating a simple
userform with one button called CommandButton and the following code:

Private Sub CommandButton_Click()
Me.Enabled = False
Me.Enabled = True
End Sub

One click on CommandButton locks up excel as described above.
<ctrl<break puts you on the line that does the userform.show, and a
check on userform.Enabled at then confirms it is True, even though the
CommandButton remains quite inaccessible.

What am I missing here?

Nick