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If the computer has the COA (Certificate of Authenticity) label with the product key for Vista Business edition, then it appears that it falls within this parameter
the only people who can assist is Microsoft, your first line of enquiry and Dell, but of course they will be unlikely to help, depending on the age of the system.
The other question of course is, and no doubt it will be a subject that Microsoft will ask
"From which source have you reinstalled XP"?
So in summary contact Microsoft