Are you positive that it couldn't be something else (like Program Files or system restore points) that is taking up the space?

Bear in mind that my last post only applies if you stored your pictures, videos, etc. in My Documents, one of its sub folders like My Pictures or on the Desktop. I'm not positive, but it also may apply if you stored them in C: or a subfolder of C:.

There isn't any reason they would have become hidden but you could try searching like namelessfengir said and be sure to check the "Include hidden folders and subfolders" check box.

By any chance is there an active account on the machine with the same name as your old account that was created after Windows was reinstalled, ie. with the same username?