“How do I convert PowerPoint pptx to ppt so that people with older versions of PowerPoint can read them? I have Office 2007 and I want people with older versions of Microsoft Office to be able to view my PowerPoint slides.”
Converting pptx to ppt is actually pretty easy, you simply need to re-save the file. As long as you didn’t do anything too wild in your PowerPoint presentation, it should just work. There is a second alternative that might allow you to keep the pptx extension, if you have features in your PowerPoint presentation that aren’t supported by older versions.

If the people you share your presentation with are open to installing additional software, the free PowerPoint Viewer from Microsoft might be a good alternative to changing the presentation. After installing the software, anyone can view a presentation with a .pptx, .ppt, .pptm, .pot, .potx, .potm, .pps, .ppsx or .ppsm extensions.
I realize some corporate environments don’t allow for installing additional software and some people don’t like installing anything they don’t absolutely need. In that case, saving the file in a different format is the best option. You will need either PowerPoint 2007 or PowerPoint 2010 to re-save the file, but if you have either the process is simple. Just open the slideshow in PowerPoint, click the Office icon in the top left corner of the application, choose Save As, and pick the PowerPoint 97-2003 Presentation option.