Use Control Panel -> Administrative Tools -> Computer Management -> Disk Management to check the thing. If it shows the partition does not utilise all of the volume, delete the partition, and create a new partition that does utilise the full volume. If it shows there are two volumes on it, then you've got a hidden partition as mentioned previously, delete them both, and create one partition using the entire volume.
If it shows the volume size as 241MB, the card is stuffed. Chuck it and buy a new one.