You may try clearing all the IE temp internet files, and clear the Temp folder in the Windows dir, and the temp dir in documents and setting /users/"your log in name" /local settings (hidden)/temp...then Reboot and try to update again....
Sometimes this will work, BUT
I suspect that you will need to UN-install SP3, Reboot a couple of times, and RE-install SP3....
I have heard of Quirky things like this happening, and after Uninstall, reinstall of the Packs, it was fixed....
Hope this helps
SeeYa
