If You upgraded Yor mother board You will need to reinstall Your operating system.
Or use some transfer software,I dont know much about that really,sorry.
If You dont care about loseing all data on Your hdd,then just reinstall windows..Fresh and clean ummmm
I also wonder if dell will let You upgrade?Ive herd the dell copy of xp wants a dell m/b or no go.
you may need a diff non dell opperating system.not sure about this..but.