I tried that, and it would not work. If a machine is bought with Vista, you cannot go backward to XP. I tried that on an HP Pavillion, and it could not be done. The machine was built for Vista, and it had to remain that way.
When I first got my new computer last year I tested vista for a few weeks and then went back to XP. It was mainly because vista was just too much of a memory hog for me.
I can't see why you had trouble in going backwards. Format the drive and install XP. There is no reason that it should not work.