Unplug the Win7 HD before booted XP install to it's HD. This prevents it from derailing the Win7 boot requiring major repairs to get 7 started and add XP.
If XP won't install you'll need to slipstream the SATA drivers in per THeog's tutorial above.
Once XP is installed, plug back in Win7 HD, set it first to boot in BIOS setup, trigger XP HD when needed by tapping the HP ESC or F9 key at boot to choose XP HD.
If this method is too clumsy, install to Win7 EasyBCD (click Download - no Name or Email required)
to add XP on Add OS Entry tab, reboot to Dual Boot menu.