Two backspaces deletes text box

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I am an experienced Word XP user who has just reinstalled Word XP on a new
laptop (old one died). This has never happened before: when I insert a Text
Box (or an Autoshape and then add text to it), if I press the backspace once,
it deletes the most recently typed character. But if I press backspace twice,
it deletes the entire Text Box (or Autoshape). The Undo feature says "Undo
Clear." The sound feedback is the same sound you get when you press backspace
within a Form Field; perhaps that's a clue or not. Does anyone know why the
second backspace deleting/clearing my Text Boxes? What setting is screwed up
that is causing or allowing this?

Thanks in advance.