|Assuming you can presently boot either XP installation, one way or another....... |
Both XP installations can see themselves as having their Windows installation on C when each is booted,
- but -
- you would have to re-install the XP installation that sees itself as having it's Windows partition on J from scratch,
(System Information shows you which partition drive letter the Windows installation that was booted from sees itself as having it's Windows partition installed on.)
- hard drive partitions that have already been assigned drive letters must either be not present, or must be HIDDEN, while Setup is running.
- if there is only one partition on each of two hard drives, disconnect the data cable to, or the power connector to, the hard drive that sees it's Windows installation as being on C, BEFORE running Setup, and while Setup is running.
You may need to change the jumper setting (Master, with slave present, to Master, single, or Slave to Master) on the drive still connected if it's an IDE drive, when you do that.
- if you have only one hard drive that has two XP installations, each on their own partition, use what is generically called a partition manipulation program to HIDE all partitions on the drive other than the one you're installing Windows on while running Setup, BEFORE you run Setup . E.g. the freeware Partition Logic if your drive(s) is(are) IDE, or the newer freeware Easus Partition Manager if you have IDE and/or SATA drives.
When XP's Setup is finished, you can connect all hard drives, and/or UN-HIDE partitions you hid, and edit boot.ini, or rebuild boot.ini by booting with an XP CD and using a command in the Recovery Console, so that you then have the choice of either Windows installation while booting.
Each Windows installation will see itself as having it's Windows installation on C when it is booted - the other XP installation's Windows partition that is not being booted from will be seen as being assigned some other drive letter.
You can change any drive letter assignment (C-Z) in Disk Management other than the drive letter for the XP installation being booted from's Windows partition, in each Windows installation, to another available drive letter - sometimes you need to temporarily assign other available drive letters to free up the drive letter you want to use when you do that.