First, right-click on the file you want to open, select Properties and click the General tab. Check the information next to Type of file and close the window. After that, if you're using Windows XP or below, go to Start > Control Panel > Folder Options, select the File Types tab, click New, type the file extension from the previous step and press OK. If you're using Windows Vista or above, go to Start > Control Panel > All Control Panel Items > Default Programs > Set Associations, locate the file format in the Name list and click Change Program. After that, use the Browse option to select Microsoft Office PowerPoint Viewer and click OK. If it still doesn't work, go to Start, type regsvr32 /i shimgvw.dll and press Enter a couple of times.