http://www.anandtech.com/guides/viewfaq.html?i=113
Just ignore the registry mullarky at the top and do the bottom route.
I just shutdown my rig, rip bits out, pop new bits in - boot off XP CD, ignore first repair prompt and continue as if doing a clean install.
When you get to just before the patritioning mullarky it will examine existing OS's and then find your existing and ask if you want to repair - Say yes
When its done its stuff you have a completely new fresh install with all your old apps/desktops and settings from the old install
Ive done this since XP first came out and have gone through over 6x mobo's etc including uniprocessor ACPI/Non ACPI, uniprocessor/SMP, AMD/Intel and Via with no problems whatsoever
Only thing to watch is your Virus scanning software which can bork the install. I just uninstall AV to play safe and reinstall when done.
If you read the MS KB linkys at the bottom of the article in full and digest you will see why i do the above as MS recommends you use this method.
Hope this helps