Where the Dell is concerned, it's a remote possibility that entering the Service Tag reference into Dell's website and asking them for the Product Key might work but I have my doubts. Lenovo don't keep such good records as Dell.
There should be save product key ,Giving detail of PC information to vendor, Can it possible
That said, the OP would only be to reinstall using a Dell OEM XP CD. It's a lost cause really - and a cautionary tale to anyone considering buying pirated Windows software.