Seems like you are not the only one who has been fooled by a company selling you a system like that
The good news, is that even we have covered this issue many times, our friends at M$ has been making a step by step manual :P
HOW TO: Change the Volume Licensing Product Key on a Windows XP SP1-Based Computer.
_http://support.microsoft.com/default.aspx?scid=kb;en-us;328874
Good luck
