Also make sure you disable you AV software
as sometimes it can interfere when istalling Service packs.
And make sure you have enough disk space left too
Why don't you install SP1 & 2 from the windows updater as this way is recommended anyway?
You say you have installed this before but to be honest your just going to be creating a lot of corrupt system files by going back and forth with the upgrading, when doing an upgrade it will always leave redundant files on your computer no matter what people tell, it just does and they are very hard to find and fix... the best way is a clean install of any OS this way they will be no possible damaged system or program files that may be lurking.
Try running a small program called Ccleaner to clean up you computer, then try again.