your data should be ok it may not boot up with the new mrd and cpu though if it does then you will have validate your o/s with microsoft. i think thats right as you have made a major change to the original installed hardware
Replacing your motherboard while still using your old hard drive will be OK but you will almost certainly encounter some driver problems on boot up unless the new motherboard is a 1 for 1 replacement. click here is a good guide and takes you through the steps of changing the motherboard on an XP system. As ^wave^ says you will need to re-validate but that is no biggy and I have done it several times.