PowerPoint is a program that is not part of the operating system
Windows XP.  It has to be purchased seperately and cannot be downloaded
(though a trial version may be available at
http://www.microsoft.com/office/editions/prodinfo/standardtrial.mspx).

Check with your school's IT group or whoever supplies your other
software.  If you have such a source, the academic price is likely to
be very competitive.  Alternatively, check a retail office supply or
computer store for the software.  It comes by itself or bundled in a
Office suite (includes other programs) and pricing is either commercial
or 'student and teacher' with different licensing for each.

You may also want to check the 'How to Buy' page at
http://www.microsoft.com/office/howtobuy/default.mspx