Okay guys, here was my full process in case anyone else does the same thing I did.
1. I had Windows 7 installed on my SATA drive to begin with
2. I Installed XP on a clean, internal IDE Drive
3. To get it to boot to Windows 7, I installed DualbootPro and installed the Windows 7 bootloader to all drives
4. It was at this point that XP would no longer boot correctly because it was giving the error message:
"Windows could not start because the following file is missing or corrupt:
Please re-install a copy of the above file."
5. I tried everything.
i.e.: Rebuilding the boot.ini file,
repairing Windows XP using the disk,
copying the hal.dll file from the disk,
reinstalling XP from scratch,
But nothing worked
6. I got on here, and SIW2 suggested that I use the auto-configure function in EasyBCD 2.0 Beta software
7. IT WORKED!
If anyone does the same thing, you can skip step 5. That step took hours of googling, restarting, swapping discs, installing updates, trying software, waiting, etc.
Apparently the boot files (ntldr, boot.ini, NTDETECT) were all referring to incorrect drive path letters (boot files on J:\ were pointing to Windows files on C:\ etc.)
Thanks again, guys!!