If you've accidentally saved over (or "written over") a PowerPoint presentation that you've been working on, you've got exactly one shot to recover your original file. PowerPoint saves temporary copies of the original PPT file in the form of hidden files in the same directory that your project is stored in. There is a good chance (though it's not guaranteed) that one of these hidden files will be a version of your PowerPoint project as it existed just before it was accidentally written over.
Open Windows Explorer and navigate to the directory where your written-over PowerPoint file is being stored. Click "Start" and then "Computer". Then use the window that appears to find the exact folder that contains your PPT file.
Click "Folder Options." Select the middle of the three tabs ("View") from the top of "Folder Options."
Click "Display Hidden Files and Folders." Click "Apply." Return to the folder where your PPT file is being stored. You will now be able to see all of the hidden, temporary versions of your project in the directory.
Open the hidden files one by one until you find the one that most closely resembles the original file over which you saved another file. If you're lucky, you may be able to find the exact file as it was before the accidental save. More than likely, you'll find a version that closely resembles the way the file was before you accidentally allowed PowerPoint to write over it.