My computer won't let me install the SP2 for Windows XP...help!!?

It keeps telling me that "The file C:\windows\system32\drivers\atapi.sys is open or in use by another application". I don't know much about computers, anyone know how to fix something like this? I'm trying to re-install iTunes, and it told me a needed the SP2 to do so, and I'm not able to install the Service Pack 2 because of that....Any help?