You can certainly automate powerpoint trough the use of its XPCOM api --
(although, everytime I looked for documentation on the XPCOM API for MS Office I haven't found it)
But, I'd suggest that instead of creating PowerPoint slides, you created Open Document presentations (ISO 26300 - .ODT files) - which are easy to undestand, parse and edit programatically -
.ODT files are the default presentation format for Open Office, Koffice, and other office software, and I think that Microsoft has come to support it by now - (although they have pushed ISO to approve their own proprietary specification of a similar format)
Anyway, editing the contents of a .odp file is trivial; the ODP file itself is a zipped archive, you unzip it, and edit the "contents.xml" file inside it -- creating new slides is a matter of copying the XML node corresponding to a slide and editing the desired text --
and pack everything inside a zip archive again.