Hi, dsp.
To ADD that old HD to an existing WinXP system as additional storage is
pretty simple. Just physically install it (cables, jumpers, BIOS settings,
etc.), then reboot and use Disk Management to assign or reassign drive
letters, or to delete, create and format partitions to fit your new
environment.
To install that HD as the BOOT DEVICE in the other computer, though, is a
whole different story - as the others have suggested. Especially if you
want to boot from the existing WinXP installation on that HD - which has
been customized (during Setup) to fit the former hardware environment.
You'll have to run Setup again, which really means to install WinXP all over
again in the new (to WinXP) computer.
RC