Digital camera SD card issue. Please help?

I recently bought a new camera and a new SD card. The camera is a Nikon S3100. After taking some pictures with the camera I'm really not happy with it, so I am going to return it. When taking pictures, I put the new memory card in it, not knowing that sharing cards between cameras could be a bad thing. Now I've tried to put the card into my old Kodak and it won't format it. A message keeps popping up on the screen saying that I need to format it, but when I try it doesn't work, it just keeps giving me the message, over and over again. I put the card in my computer and formatted it on there thinking that it might help, by removing all the data from the Nikon, but the Kodak still won't read it. Incidentally, the camera still reads the old SD card, so I know it's working, and am assuming that the problem is due to using the card in the new camera. Does anyone know why this is happening? And, is there a way to fix it? I'd hate to buy a new 8GB card when I've already got a brand new one.