If you have a long animation looping animation that you want to be able to pause and then resume, there's an easy way to do so with the keyboard. Press the 1 key, which puts you in the mode where you can type in a slide number to jump to. In this mode, all animation is paused. To get out of this mode without jumping to another slide press the ~ key.
There are lots of keyboard shortcuts in Powerpoint; press F1 to get a list. Note that one of them is listed as "pause" (S key or + key) but this doesn't actually seem to work in all the situations that the trick described above does.
Tested an known to work in PPT 2003.