I had a similar sounding problem with my Vista-loaded machine which came with an XP 'downgrade' which I installed pronto. That worked fairly well except the ruddy machine still 'remembered' certain odd things and refused to recognise / accept the installation of certain software in spite of using original, genuine installation CDs, complete with registration keys etc
I didn't need to in the end but what about blacking the HDD [Format C:] then try installing XP ?
CS