Windows computers use files with the SCR extension for screensavers, but PowerPoint can't save to that file format. If you want to use a PowerPoint presentation as a screensaver, you'll need software to convert the PPT file to a SCR file.
Download a conversion tool that will convert PPT -- or PPTX -- files into SCR files. Some options available are PPT to EXE from Boxoft.com, PPT to SCR Converter at Winsite.com or Screensaver at FlashDemo.net. Once you've downloaded the file, double-click on it to begin the installation process. Follow the on-screen instructions to install the software on your computer.
Open the newly installed program. Depending on the software you might need to click "Add PPT" or "Convert to Screensaver" as the first step. Consult your software manufacturer for specific details. Once you get to the point where you can add your PowerPoint file, browse to the place on your computer where the file is saved and then click to select it. Click "Open."
Select the options you want for your screensaver. Some software, like the Boxoft PPT to EXE converter, allows you to add your own transition effects and music. Other programs will use the transitions and audio within the PowerPoint file.
Build your screensaver SCR file. Note that Windows will typically look for an SCR file in the Windows\System32 directory on your computer. Save the file to that location so that you can easily load up the screensaver you've created.
Right click any blank space on your computer's desktop and then click "Customize" to access the screensaver settings. On older versions of Windows you might need to click "Screensaver" after right-clicking.