If it's a Dell machine that came loaded with XP Home in the first place. It will have a hidden partition that you cannot format using ordinary methods. I am not sure but I think that the discs that Dell provide take this into account and I think you should contact Dell for advice. I have a Dell 4300 and recently upgraded from Dell loaded XP Home to XP Pro, but I used a full XP Pro disc not the one that Dell would supply with an XP preloaded machine. I have had no problem at all with the full version of XP Pro.