COMPUTER HELP !!!!!!!!!!?

i rebooted my system and installed a different program , it used to be windows xp pro , now it windows xp home and i installed all the updates but when i install xp sevice pack 2 it downloads , but say c:\windows\system32\drivers\atapi.sys is open or in use of another application please close all apppliations and try again , i already tried clicking retry and closed all other programs , PLZ HELP !!!!!!!!!!!!!!!!!!!!!!