repeated ITunes store authorization?

When using usb plug with IPhone 4S to pc, continue to have to authorize computer each time with ITunes store in order to snyc. Brings up old email address. How do I delete old address I no longer use. But, when I do authorize and plug in usb again I have do the procedure all over. Uninstalled ITunes and reinstalled but still have issue. Tryed Deauthorize computer and authorize again but every time I reconnect phone have to authorize again. Thoughts?