Hi i need to downgrade from windows vista to xp on an advent 5401 laptop?

I'm having serious problems trying to downgrade vista to xp on an advent 5401 laptop, the laptop won't even boot from the xp disc, i've changed the boot device order in the BIOS to boot from cd rom and i'm aware that i need to change the hard drive type before it will recognise the boot disc but i have tried every hard drive type option available on the BIOS screen and it still won't recognise the boot disc, if anyone has resolved this issue please could you let me know how you did it, many thanx in advance