Messed up running two os' in my computer how do i fix it?

alright for a little background, i had vista ultimate SP1 then i
upgraded to a sp2 build and today i decided why not install and run sp1
also, to see if my wireless card would work with sp1 because it didnt
with sp2, so im an idiot and i forgot to partition my HDD so now i have
two os's on one HDD how do i fix it. Right now i half all my old stuff
tucked away in a Windows.old folder, now i want to delete my sp1 that i
installed over the sp2 build and just go back to the way it was before i
installed sp1, how would i be able to do this without just starting over
from scratch because i had a lot of programs, games, videos, ect (about
650GB of stuff) that i dont want to have to go back and download all
over again. Any help at all would be appriciated.