If the files are encrypted by XP, you probably have little hope, unless you
deal with a professional ($$$) file recovery firm.
The fact that you were able to get some files via Vista leads me to believe
that they were not actually encrypted. Rather, this appears to be a case of
file "ownership", which is a different thing altogether.
You might want to try using a LINUX "live" CD to boot the PC. Being a
non-Microsoft operating system, it ignores the XP permissions. Modern LINUX
can read NTFS, as well as FAT32, the two partition formats that XP uses. I
suggest KNOPPIX, since it tends to automatically detect most hardware,
including SATA drives, video cards, etc. Download an ISO image from the
following link. I suggest the CD version 5.1. Do not try 6.0, as that is
still experimental, and has limited hardware support. And, you do not need
the DVD version for simple file recovery.
http://www.knopper.net/knoppix/index-en.html
Burn the ISO to CD, then boot the PC to which the old hard drive is attached
(e.g., via USB, or internal wire). You should come to a KNOPPIX desktop,
with all internal hard drives indicated as icons with names like "hda1".
Use one of the file managers to view the hard drives. You should rapidly
find the old one. Then, work down through directories until you find your
files. You should be able to do a copy and paste form the old drive to
either the internal drive or to an external drive.