First, the ones that you bought in the first place should have come with necessary passwords, unless they were stolen. Contact the seller. If the seller can't provide them, you have a moral dilemma.
Second, an XP disc that came with another machine is both legally and technically not installable on your machine. Technically, because it will only have the drivers for the Dell it was meant for. Legally, because the OEM license is limited to the machine it came with.
Contact the sellers. My best guess is that you have been sold stolen or against-license material. You could always purchase a legitimate, retail XP disc and install it on - one of them. You would need two discs for the two of them.