Here is the downloadable utility from Microsoft that will allow you to open Office 2007 formatted documents, with the x on the end of the file extension, in Office 2000, XP or 2003. This is a nice band-aid to a huge problem that occurred when MS released a non compatible file format for the Office 2007 suite of apps. I have heard that there is more compression in these new file formats, so a PowerPoint file would be significantly smaller but I haven't tested that myself yet.
I assume that there was a great reason for introducing this new set of formats that would off-set the pain it is causing, but I can't think of any.
Anyway, you can download the Compatibility Pack from http://www.microsoft.com/downloads/details.aspx?FamilyId=941B3470-3AE9-4AEE-8F43-C6BB74CD1466&displaylang=en
or you can use http://www.zamzar.com to convert your files.