I presume you're using the 200Gb as the only drive, albeit partitioned. The old 40Gb is not still your C: drive, is it?
I also presume that you've backed everything up!
You'll need another computer or have access to one to do the first bit:
Get the latest bios from your MB makers site. Download all known updates to bring your version of XP up to date, save these on a DVD etc.
Once you've got all this stuff and if you're confident in what you'll be doing you should upload the latest bios first, then try Fdisking the drive first and reinstalling, check the size indicated in the bios screens. Then split the drive into three partitions
C: System
D: Applications
E: Data
Then load all the XP updates etc and then try the repartitioning to the utilise the full size of the drive.
If it still fails then you may have to run the fmbr command to wipe the Master Boot Record on the drive and start afresh with the fdisk and XP install.
Hope this helps.