The first thing to do is scan your computer to make sure you are clean of all malware, something that can prevent SP2 from being installed correctly. Next, I would run SFC to make sure your system files are in order. Finally, I would click here to download the SP2 installer. Just ignore the alert telling home users not to download it and use Windows Update instead.
Hope this helps,
He how can i upgrade Windows service pack 1a to Windows service pack 2. And how do i do it. I tried microsoft update and it didn't work. i don't have windows service pack 2. Please can any body help.