Aside from the possibility of a really weird hardware problem, the likely explanation is that somehow the 200 GB disk unit has a single 10 GB Partition set up on it, and that's what you are using as a "drive". The rest of the space simply has never been set up to use. You can see the details of how all of the disk is arranged in the lower right pane of Windows Disk Management. Look there and tell us what it says. I suspect it will show you a small block one drive of 10 GB plus a very large block called Unallocated Space. Let us know.
Also tell us what is on this drive. If it is your only drive and the space in use is really your C: drive, changing this requires particular tools. Other situations require simpler procedures. No matter what, I can't quite imaging having Win XP Pro installed on a 10 GB drive. So, what drive letter is it? How many hard drive units do you have? Which has your Win XP Pro installed on it?