One thing you could try is flashing the BIOS.. perhaps it is somehow corrupt or has a setting wrong - flashing it to the latest version should, at least, restore it to a default failsafe setup that should work.
Also, given the errors could relate to the hard disk or installation of XP itself, have you tried booting off a bootable CD, like a windows setup CD or better still, a live CD like BartPE or something similar.. that should help to at least tell you if the thing works ok in a different software environment.
Removing or trying different memory is definitely a wise suggestion - it is the most likely cause.
One other thing you might consider, depending how technically capable & flawless you are at disassembly/reassembly of complex electronics, is the old army gun approach - if you're pulling something apart and putting it together properly regularly, you are more likely to have the thing in good working order... perhaps something has come unslotted that this approach would resolve?
That's my 2c. If I billed you, trust me, it'd be a lot more.