If I understand correctly you want to backup your C drive which has 13GB used. If you do not have any other large volume backup device attached you can only backup to C, A. To backup your curent C to floppies would take 8667 discs (they only hold 1.44MB)
I guess the only way would be to create a folder on C called Backup and allow the utility to store the backup there and then burn that Backup folder to CD's, but even this would take 20 CDs or 4DVDs.
Another possibilty is to use NT Backup to save your Documents and Settings only, although you will have to reinstall your OS and other programs at least all your personal stuff is saved.