Hmmm there's something i can't understand >.< sorry...
Your computer came with XP, then you ordered the vista disk, you get it and install it, then updated it to SP1 and now you get this problem?
If so, then you should be abble to uninstall the SP1
i mean, is not the same repair the instalation with SP1 and without SP1... it's never the same or at least it never worked for me with xp to repair xp with sp2 with an old disk without sp2 ... but, with vista idk.
I'll tell you something that may help you, when i bought my original license of vista home premium, it cames in a CD and was an old version of vista (without SP1) but the SERIAL was for vista home premium with SP1 so, with it there was a paper with this instructions for the resealler: "In order to install properly the os you should provide the customer with a new image of windows vista home premium sp1 disk" and my provider just ordered again to microsoft the new disk and in a week or so i get the dvd with sp1, i guess your resealler can do the same for you
give it a try like that
it worked for me here where the sun don't shine xD
/ 
