If you have office, wouldn't you already have PowerPoint?
THe easier solution would be to simply install powerpoint into the Office folder you already have. It should be easy enough to do. If you start the installer you should be given the option to update your current office suite and add programs that weren't there before.
(even if you have Office 2007 and you want to install PowerPoint 2003 you can still do it, the installer will simply install Power point in a folder called Office11 while the 2007 files will still be intact in a folder called Office12, or at least a folder name similar to those)
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