Yes you do purchase the License which is joined with the Serial. But as you said you work for a refurbish company, which means that the customers should already ahve the XP CD at their disposal. If not they can contact their original PC manufacturer to get a copy.
@worrwhorre - as you can do this using the serial from the Dell machine is what makes it illegal. Unless you bought a new License it would make your OS on that system pirated.
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