If I understand you correctly, you installed XP on your new drive (as master) with your old drive, which contains XP and was used previously as your boot drive, installed as the slave. If this is correct, when you booted-up your master drive was established as the boot drive and assigned an ID signature which includes the drive letter (C:, I presume), and your old drive was given a new ID signature and assigned a new drive letter (D:, I presume), making the old drive un-bootable.
If I am correct so far, you can remove your master HDD and set it aside, then install your old HDD in the master slot, and boot your computer to a Win98 startup disk (which you can create, see bootdisk.com) and run the command fdisk /mbr.
The fdisk /mbr will rebuild your master boot record and zero-out the HDD ID signature. Remove the Win98 startup disk and reboot, which will force WinXP to recreate the HDD ID signature and reassign the drive letter (C:, I presume).
Your old drive should now be bootable, and you can uninstall Go Back. Then take out and set your old drive aside, and install your new drive back into the master slot, and boot the system to the Win98 startup disk and run the fdisk /mbr command again. Remove the disk, and reboot. The master drive will be reassigned as the boot drive and should have the letter C:. Then install your old drive into the slave slot and reboot the system. With both HDD's installed, and with the new drive as the master and assigned the drive letter C:, the slave drive will be reassigned as the non-bootable D: drive.
Note: Do not look for an affirmative response when you run the fdisk /mbr command. You will immediately return to the command prompt without any notification that the command has been successful. Also, this cannot be done with the fixmbr command from the XP installation CD. While fixmbr will rebuild the master boot record, it will not zero-out the HDD ID signature which is what you need to accomplish in order to make your old HDD bootable.
