Don't know if this differs, but how do you download, directly from camera or via a seperate card reader? Try both to see whether there is anything accessible on the memory card...
Did you format the disk in camera before photographing?
I had this thing with a friend's datatank. While full with photos, she couldn't get access to it with her laptop, my desktop pc saw it as being unformatted and my laptop (all Windows XP SP2) had no problems with it!
Fortunately we were able to save the memories of a 3 weeks trip to India!
So, try another computer too...
Good luck!