Change Desktop Icons Windows 7

I have two computers that I upgraded from Windows XP to Windows 7 using the Easy Transfer Wizard. Everything seems to have properly transferred but there is one problem with both that I had not encountered before. On the desktop on one computer the user had saved several PowerPoint presentations. They are there but rather than the usual PowerPoint icon there is a very small thumbnail like icon that looks like the first slide of the presentation. On the other computer the user had saved several pictures on his desktop and those icons look like small thumbnails of the images tather than the icon from the program in which they were saved. How do I change these icons to the program icons?

Thank you!

Robert