If you want to have a dual boot install, the normal way is to set the SSD and second priority and let the install use the active partition on the XP drive for the boot files. A Legacy install (non-UEFI) will put the boot files on the primary drive, then install the OS where you tell it to.
If you wanted to install Windows 8.1 on a drive as an self-contained install, where you would not need the XP drive to boot, you would set the SSD as the primary drive in the bios. Then it would install the additional System Reserved partition which holds its boot files and Recovery Files, followed by the OS partition. Installing this way will require special steps to put the two OSes in a Dual Boot configuration or you can change the priority of the drives in the bios and the system will boot to the first active partition it comes to, which would be the Windows 8.1 or XP, as you so choose.
The SSD need you to have your system set to use the AHCI SATA controller to get the most from it. Is XP currently installed using that controller or IDE?