Memory full message, but no images

My son has had this camera for a long time and it has been very good, but after some recent rough use at the skate park, it has begun to display the message "Memory full" at power on. However, when we go to view images, the message says "no images." I have used Windows Explorer to see if the camera can store images and, indeed, I have been able to copy images to the camera (using it like an extra flash drive) and delete the images. However, when I copied images to the camera usign Explorer and then tried to see the images using the camera's menu system, I got the same "Memory full/no images" messages. There is no SD card in the camera. I have tried to remove the batteries and re-start the camera, but after re-settign the date/time, still get the error messages. I also used Explorer to "re-format" the camera's memory, which it did. However, the messages continue. I have not been able to figure out how to view the internal "working" memory of the camera, but I'm not sure even if I could that it would do me any good...it feels like I need to somehow re-set the camera's "operating system" if that was possible. Any easy answers out there? Thanks! - Andrew and his pop