You don't say anything about your PC and so it is difficult to answer you question. However, in order to run XP your PC will need to have a least 512MBytes of RAM and a reasonably fast processor. If it is currently running ME it is unlikely to have either. Next you would have to buy a copy of XP. By the time you go to all of this expense, you may be better off buying a new computer.
t, February 2008