To do it quickly you need to drag the alignment tools into your tool bar
along with the "Relative to Slide" then just click the align horizontal and
align vertical with your content selected.
Just to expand a little on John's answer:
2003: Click the 'draw' drop down on the drawing toolbar -> align or
distribute
2007: Picture Tools (or Drawing Tools)/Format tab -> Arrange group -> Align
(or add them to your QAT).
If you need any more help be sure to tell us your version