Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats.
Just visit this Page, from Microsoft.Com
it’s about 27.1 MB..
i am have installed this package on my XP Office.. and it works fine with me 
Thanks Microsoft.. instead of paying all these money for da office 2007 
it’s look like a Converter.. that convert PPTX “as XML File” to PPT using the installed office (whatEver 2003,XP,97).
Just Double-Click on any Extension which is supported by Office 2007 only like PPTX and DOCX.. and u will get another file with large size “in most cases” as PPT or DIC.
and Viva “Open XML” Technology 
