Word XP Freezes When Autonumbering

Repeatable problem: I'm working in Word XP, and letting the document automatically set paragraphs to "a.", "b.", "c." and so on. But on one of them, I need to change it to move it up one level (e.g., from "e." to "4." with a reduced indent, becauseI've been on "3." and now I'm done with "3.").

It seems to me that before, I just clicked the "Decrease Indent" icon on the Formatting toolbar, and it worked. But now, every time I try it, the Word freezes. In fact, I think the entire computer freezes, if I recall, and I have to use Ctrl-Alt-Delete to get to the Task Manager and kill Word.

KEY POINT: It doesn't happen on my Dell Latitude---it works fine there. It only happens on my Sony Vaio PCG-SRX87.