help!!!

i have just bought a computer complete with windows xp,
the man that i bought it from told me i could take over
the ownership of windows xp and other documents he told me
how to do it but it doesn't work. its is registered in
his name and all files are in his name i cant change the
file or folder names and i do not know any way to change
the ownership of xp etc or files or folders, please help!!!