Presentations lose animations in Mac PowerPoint X / Windows PowerPoint 2000 and previous
PowerPoint 2002/XP (Windows), PowerPoint 2003 (Windows), and PowerPoint 2004 (Mac) use an entirely new animation system.
Don't use the new animations if you plan to view the presentation in any older version of PowerPoint for Windows or Mac.
If you know your Windows PowerPoint 2002 or 2003 presentation will be shown in any earlier version of PowerPoint on Windows or PowerPoint on Mac (X or earlier), choose Tools, Options from the menu bar, click the Edit tab, then put a checkmark in each of the three boxes listed under "Disable new features". That way you won't be able to accidentally use any features that won't work in other versions.
Using the Compatibility Report feature in PowerPoint 2004 (Tools, Compatibility Report) will allow you to optimize your presentation for whatever version of PowerPoint the other computer is running.