A proper Arr'd copy of Windows should have no problem getting service packs. Sorry to ask a stupid question but have you -tried- downloading the service packs through windows, or are you assuming you can't get them? If it doesn't work you need a better copy. Since I'm feeling helpful, here ya go:
Link to TorrentI haven't used that specific copy, I don't remember where I got mine, but according to the comments there the activation is already included with the install, so it shouldn't turn up as non-genuine. If you have to go hunting or you don't like the linked one, any copy that's pre-activated should be fine, and the ".iso" format is the easiest to get burned onto a disk. (Other formats sometimes require a specific burning program, which you could also Arr, but I'm reasonably sure you can use .iso with the basic programs everyone tends to have.)
I don't think it's possible to update your current installation with the new disk, but I'm not very experienced at all in that regard, so maybe someone else will chime in. But I do know that if you backup your files, reformat, and reinstall, this will take care of it.