You're very welcome.
Sorry that didn't work.
XP should have installed with the controller set as "Separate IDE Controller."
I am not familiar with the HP Workstations hardware configurations, and unfortunately I don't know what drivers you need to load to get XP to work.
My recommendation is that you post your question on the HP Business Support Forum -- xw Workstations section and maybe someone there can be of better assistance than I can.
http://h30499.www3.hp.com/t5/Workstations-z-series-xw-series/bd-p/bsc-272#.UeaT1dco7Gg
I just know that the problem you are having can be solved by adding the right SATA driver either with a USB Floppy disk drive at the F6 prompt or by slipstreaming the drivers.