System kept crashing or would not load winxp at all(pirate i thnk) new xp loaded still wont work?

i think my daughters pc (second hand) had pirated windows xp installed and kept crashing and finally would not load at all i bought a new xp and loaded it (put disk in and followed instruction) but it is just the same..what have i done wrong..i am not all that pc literate so any help in plain english would be appreciated..thx