Hi Chris,
Sorry to hear of your problems. Am I correct in saying that it is your drive with XP on that you tried to install the game and with which you are now having problems?
Does your system recognise this drive? Boot up and enter your BIOS (usually by pressing Esc, Del, or F2 - see your manual for instructions). Is the drive listed? Since the drive is in a caddy and is therefore designed to be removeable, it is possible that the connections inside the caddy might have worked loose, so check them. I suspect that the drives are IDE, so this might explain your problem with the other drive in your system. Check the jumper settings on your hard drives - the one in your system (which you have currently disconnected) should be set as SLAVE SL and the ones in your caddy as MASTER MA (assuming, of course, that they are connected to the same cable with the internal drive on the middle connector and the drive in the caddy connected via the end connector. If it is the other way round, simply swap the connectors or, if that is not possible, swap the MA and SL settings on your hard drives. Whilst you're at it, is the internal drive recognised in the BIOS with no other hard drives connected?
If these drives and your optical drives are both IDE, then make sure that the jumpers on your optical drives (if you have more than one) are set to the appropriate setting for their position on the cable, and that they are connected to the Secondary IDE connector on your motherboard, with the hard drives connected to the Primary (again, refer to your motherboard manual).