Just for general information, the key code on the bottom or other places on
most of the big manufacturer's computers (Dell, Gateway, HP, etc.) are not
the same as the key on the installed OS. They clone many hard drives with
one key and if replacement is needed by using their rebuild setup then the
key on the sticker is used.
Read the sticker and it should tell you what OS should be in the system.
You know that it will be a Dell OEM version. If it is not the same (XP Pro
or Home) then you have, in all probability, a pirated version of the OS
installed on your used system. When you purchased the "used" system, did
the seller give you a hologram Windows XP install CD? If not you should be
contacting them again for it or for them to replace it with a legal OS.
Anyone selling a used computer with an OS installed is required to also
provide the installation media that came with that OS.