Unallocated partition

I have done a complete reinstall of Windows XP on my grand children's Sony
desktop which is about 5 years old, there are no original support discs.
The system is up and working fine, however when I look at Disc Management
I see the following, Disc O Basic 111.79gb on line -- "c" 27.95gb NTFS healthy
In black 83.84gb unallocated.
I have searched the Threads and Posts and found and used the following,
Command Prompt -- diskpart --list volume --select volume --extend, but when I get to this point the message is ( please make sure the volume is valid
for extending) what am I doing wrong or need to do? I would rather not have to do a complete reinstall and all the updates again, but want to extend
the "c" partition to the full capacity of the drive, is this possible. Mike Devon

 

Here is a link to a kb article that should help.
How to use Disk Management to configure basic disks in Windows XP

 

To amdace and PeteC Thank you for your replies.
I printed the information from the link and read carefully, but was still unable to extend the (c) partition, it would seem that the best solution is to make a new partition (D).
I appreciate your help, Thank you. Mike Devon

 

To PeteC Thank you for that explanation, I now understand that I can not do what I wanted to do, hence your suggestion to make a new partition.