You can try installing XP onto a partition you shrink on the other HD. Be sure to unplug the Windows 7 SSD first so it isn't interfered with by XP install. Partition or Volume - Shrink
You may find that XP installer cannot see the HD since it's so old the installer has few SATA drivers
. In that case you must slipstream the SATA controller driver
for XP into the installer. But you can try it anyway.
After XP install, plug back in Windows 7 SSD, set it again as first HD to boot in BIOS setup, boot XP HD when needed using the one-time BIOS boot menu key at boot.