would agree about posting the error message. I'd guess (until/if that appears) that its the SATA drive setting. its probably set to AHCI for vista - but xp won't be able to see it in that mode (without additional drivers) - so need to change it in the bios to IDE or legacy mode.
Providing the model of the Dell would help. many of them do not have driver support for earlier versions of Windows other than what was installed. Many Vista machines had no drivers for XP. Check the Dell site for XP drivers yourself and see what it says for your system. As earlier stated the AHCI mode prevents XP installs. If the BIOS does not have settings for "compatibility" or "IDE" mode then you will not be able to complete the install.
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