Open in Explorer (Start,Programs,Accessories,Windows Explorer) Navigate to your file. Tools/Folder Options/View Untick Hide Extensions for Known File Types click OK
Select the folder your file is in, in the left panel. Select your file in the right panel (click once till it is highlighted) Click it again and a box will appear round the file name with an edit cursor inside and the name highlighted. Click once again so that it is no longer highlighted. Use the backspace key to delete the file extension, then retype it as what you want to change it to. Click elsewhere to finish.
Changing it to a gif or jpeg extension will not change it to a gif or jpeg file (or make it read able as such to any software) any more than slappign a Ferrari badge on my car will make it go faster and look good....
Try installing Powerpoint Viewer to view this type of file. click here
Depending on which version of Powerpoint you are using, and if you loaded the option from your installation discs (Office 97 onwards I think), go to FILE>SAVE AS> and in the bottom file type drop down box you will find the option to save it as a Powerpoint Presentation .pps