Circle the keywords
Lets say you have a slide filled up with a number of points. During the slide show, you want the audience to take note on certain keywords. One way you can do is to draw a ring (a circle without fill color) around the keywords and animate it in such a way that the ring is being drawn around the keywords automatically during the slide show.
This can be done in PowerPoint, with the help of Wheel effect. Using PowerPoint 2002/XP and above, go to the Custom Animation (Click Slide Show > Custom Animation).
Select the ring on the slide.
Now, click Add Effect > Entrance > Wheel. (Click on More Effects if it doesn't appear on the list).
Select the added Wheel effect.
You will see that the default number of spokes is set to 4. Change it to 1 and we are done.
You can then adjust the speed to your preference.
You can also take a look at my Traditional Countdown tutorial which is similar to the above tutorial. Traditional Countdown tutorial also includes 2 samples for you to take a look at:
http://pptheaven.mvps.org/tutorials/countdown.html