Insert the SD card into your PC's card reader. Open a Command Prompt as administrator. Type "diskpart" and hit RETURN. When diskpart has loaded, type "list disk" and hit RETURN. Look for the drive entry that's your SD card (compare the capacities) and note which disk number it is.
IT IS CRITICALLY IMPORTANT THAT YOU SELECT THE RIGHT DISK NUMBER BECAUSE YOU WILL BE COMPLETELY ERASING THAT DISK IN THE NEXT STEP.
Type "select disk #" with # being the disk number of your SD card and hit RETURN. Type "clean" and hit RETURN. This will remove all partition and formatting information from the SD card. When it's done type "exit" and hit RETURN, then close the Command Prompt window.
Eject the SD card, then reinsert it and Windows should prompt you to format it.