Just use the camera to format the card (after copying everything to the pc!!) Most mobiles and cameras include a tool to format your memory card! This will completely erase the card restoring it to its full value (512mb?).
Failing this you might want to format the card in your pc! insert card into memory card reader then rightclick drive icon and select format! make sure you format in FAT and not NTFS.
SniperJunkie
June 2008