You might have installed XP using the Fat32 option instead of NTFS. If
that's the case and if the other drive is formatted NTFS then the
reinstalled system can't see the other drive. You need to reinstall XP as
though it were a new install and this time when you format that drive, use
NTFS. Don't format from a Windows 9x boot floppy, do it during XP setup as
follows:
Boot with the XP CD in the drive. When you see the message on the boot
screen to "Press any key in order to boot from the CD," do so.
After loading drivers and files, you should be taken to a screen with
the following:
To Setup Windows XP now, press ENTER.
To Repair a Windows XP Installation using Recovery Console press R.
To Quit setup without installing Windows XP, press F3.
In your case, press ENTER.
Agree to the License agreement by pressing F8.
You will then be taken to a screen with two options.
To repair the selected Windows XP installation press R.
To continue installing a fresh copy of Windows XP without repairing,
press ESC.
This will bring you to a partition map where you
can delete, create and format partitions.
Select the drive you wish to format, delete the partition, then create a new
partition, format as desired and continue with XP installation.
If this resolves the issue, check out the third line in my signature. Hard
drives crash, hard drives fail and if yours does, then what happen to that
100GB of data?