ghost your hdd (copy it, so that if you completely phail, you can fix it)
install vista, then find each and every single driver for your PC
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First: contact your dell representative to know if your systems windows licence supports downgrading, if so ask for the product key, if no, you must buy a copy of windows XP.
Install it (it's easy, but you could find guides about it)
After installing, go to the dell page, then support and then enter your pc model to download the drivers, sellect windows xp as your OS and you are done!