In general, disconnect the power lead from the Win 7 drive and then install XP with only the 2nd drive powered on. You can then use the BIOS to select which drive to boot from.
I will say that Haswell is a poor choice for XP as many devices will have no drivers, such as USB 3.0. A Sandy Bridge Z67 (2700k) system has full XP compatibility, and Ivy Bridge (3770k) works too as long as not using the IGP. Z77 chipsets require the Fernando AHCI driver. The fastest possible would be an X79 S2011 system with Sandy Bridge-E 3970x or Ivy Bridge-E 4960x 6-core with HT. These also need the Fernando AHCI driver.