Good news! A presentation created in PowerPoint 2007 is compatible with earlier versions of PowerPoint. You just need to choose the option to save as a PowerPoint 97-2003 presentation in the "Save As" list of options.
Click on the round Microsoft logo button in the top left corner. The easiest method is to hover your mouse over the Save As option and choose "PowerPoint 97-2003 presentation" from the selection.
A small note of caution though. If you used features that were new in PowerPoint 2007, those features will not work if you save your presentation as a PowerPoint 97-2003 presentation.
Beginner's Guide to PowerPoint 2007
10 Part Tutorial SeriesCompare Prices to purchase PowerPoint 2007
Related Article - My Review of PowerPoint 2007
• What's New in PowerPoint 2007?