The new release of Powerpoint 2007 apparently creates a format with the extension, .pptx, which doesn’t open on the Mac, even in most current version of Powerpoint. You need to download some integration tools to use it even on the PC on the earlier versions of Powerpoint. Those are here. Download and install them to your PC, then restart.
But wikipedia confirms that because of some of the media types PPT can include, some presentations will not be viewable/workable on the mac.