I had this happen as well... gifs weren't showing up... I messed around with the "folder options", assigning the extensions to IE, and eventually got it to work on two occasions. I'm not sure what causes that (as it showed gifs FINE when I was on-line - if the page was on a webserver, just not on my machine). And I forget exactly what I changed (not sure if I ever knew, heh).
I do believe that it happened once I installed a graphics program though.