It's a freeware, and it works well.
PowerPoint Viewer lets you view full-featured presentations created in PowerPoint 97 and later versions with full fidelity. This viewer also supports opening password-protected Microsoft PowerPoint presentations. You can view and print presentations, but you cannot edit them in the PowerPoint Viewer. You may use the fonts that accompany the PowerPoint Viewer only to display and print content from a device running a Microsoft Windows operating system.
PowerPoint Viewer registers itself with the .ppt, .pot, and .pps file extensions only if a version of PowerPoint is not installed on your computer. If registered, double-clicking on these file types will launch PowerPoint Viewer.