When selecting two or more objects to align in PowerPoint 2003, how do I specify which object should remain stationary so that the others will align with it?

So far, regardless of which object I select first (or last) the results are the same. Align Top always moves all object to align with the top of the topmost object on the slide, etc. How do I align with the top of the lowest object or with the right edge of the center object, etc?

Thanks