Change dell inspiron 4550 from windows xp to windows xp professional

I have a Dell Inspiron 4550 which is around 7 years old. I had a technician come over last year to upgrade it. I am not a techie so I don't really know what he did but it cost a lot. My 4550 is loaded with windows xp but at the moment is painfully slow. I would like to completely clean up my computer and start afresh by loading the Windows Professional SP2 pack I just bought. Can someone explain to me do it please? I do not know how to clean up my computer from all that it contains now. Thanks

Rami


BTW: When I say slow I mean to internet access otherwise the desktop is fine. Thanks