use msconfig to shut it down before you try to load SP2.
Have you made certain you have no malware present on the computer?
Installing SP2 if there is malware present can create havok!
Run Windows One Care Free Scan first, before youinstall SP2.
Go to Windows Live Onecare Free Scan
It will say "Get a free PC safety scan"http://safety.live.com/site/en-us/default.htm
Make sure you click "Full Service Scan"
in the middle of the page and not the "Try It Now Free" on the right side.
Allow it to download an Active X component.
Choose "Complete Scan" in the window that opens
Do not click on anything else that offers you a free trial or to sign up if you live in the US.
Allow it to scan - it may take quite, maybe two hours or so depending on how big your hard drive is and how fragmented your registry and drive are