If each line is in it's own layer, you could just create a .jpg from ArcGIS as you progressively check the box to display each additional line/layer. If you have 5 lines that you're wanting to for your PowerPoint, you would have 5 .jpgs (or 6 if you want a starting graphic without any of the lines).
Using the layout view might help you save uniform .jpgs. Remember to not adjust the zoom/pan as you create each jpg so that it's easier to overlay each graphic in the same place. It will look like the lines are animated.
Personally, I don't use animations very often, because of the potential downfall of using someone elses computer. Animations aren't always interpreted the same by each version of PowerPoint or operating system. I usually create a series of slides, with progressively more information on each slide. It looks like I'm using animations, but it saves me the headache the headach of not being able to show each time step.
I'm not saying that this is the BEST solution, but it's a possible solution.
