Disconnect from the net. Turn off all AV programs. Close all running applications. Try again. If no go, download SP2 and burn it to a CD. Try from there.
I've downloaded SP2 several times and I keep getting "This installation does not support your operating system?s current Service Pack version". My operating systems is XP Home 2002 Service Pack 1. At one point when I tried to download SP2 again the Microsoft page said SP2 was already loaded. It is still showing SP1 and the automatic updates continues to tell me to load SP2. Internet Explorer is V6.0. I've shut down and restarted several times. I have no idea what to do at this point.