Laptop keeps telling me to Format SD Card?

I've recently bought the nikon d3100 and i'm still getting used it it, the other day i was out taking photos and wanted to look back at them on my laptop. However, when i put the sd card(4gb) in it said it needed formating, and i knew that meant erasing everything.

So i figured there's be some wiz programme which would help me get the photos off it so i left it for now, just tried my (2gb) one and it says the same thing. There's nothing on this card so i clicked 'format' and it said 'unable to complete format' so i looked up how to do it in my camera, and i did it.

But it's still coming up when i put it into the laptop.
And i have used this 4gb card before, when i first bought the camera i took a few test shots of my dog on that card and it worked fine then.

Does anyone know what's going wrong?

I would first use the USB cable connected to the camera to get the pictures off.

I would try another computer to see if the pictures can be retrieved directly from the SD card

Do the pictures show on the camera with the SD in camera? The SD card is probably not the problem since another SD card does the same thing in the computer. Once this happened to me and I believe I rebooted my computer and it read the card