I have just spent the last week with exactly this problem... I even went to the extend of replacing both motherboard and CPU, swapping in and out with RAM modules, CD/ DVD drives, graphics card, floppy drive(!), cables, running memtest86 on all my RAM, running different FSB and memory speed, using more than one XP setup CD, holding my breath, closing my eyes and praying. No joy... but the process did mean that I eliminated just about everything except the hard drive and the power supply.
Almost out of desparation, I had noted in a thread elsewhere (a modding community) were discussing tweaking the power and the tolerance that the RAM chips could manage. So I up'd the voltage (through the BIOS) to the DDR RAM only, same problem but less often. Up'd the voltage for all component by +5% and it's working. Still got the problem once during setup, but hit retry and it sailed through. No issues so far, fingers crossed.
Although there is an obvious risk associated with increasing the power, if you choose to take it, it seems to solve this issue.