Just trying to cover all possibilities.
Try defragging again if you haven't done it in a while. Before you do clean out the temp directories including the one under the Windows folder.
When it starts to slowdown (I would test this at home rather than during a show) open the taskmanager and check performance to see how much memory is being used/available. If it is close you may want to increase virtual memory.
What size is your internal hard drive and what percentage is free? Microsoft recommends a minimum of 25% freespace. Many of the older laptops had small internal drives and after a while XP will fill them up if you don't keep it in check.