From your most recent post, it appears as if you are using DOS commands without using DOS parameters.
If you want to use the "FORMAT" command from DOS, you must use the correct parameters. To see what they are, from the DOS command line type, "format /?"
It could be you have a 19GB formatted partition on your hard drive and another unformatted (unseen) partition of 20+GB.
I am not familiar with centos, but you might want to check the centos site for information. I believe I read in one of the centos formats where one individual had a problem similar to yours. He had to delete the centos partition using "FDISK" from a Windows startup disk.
A good Linux-based partition editor is "gparted" which can be found at http://gparted.sourceforge.net