Wait, what? Why would you possibly need to reinstall the game if you lost your SI disc? My SI disc is sitting in a corner, collecting dust. You only need your Oblivion disc to play, even with SI installed.
Well for a back up, I copy the oblivion folder to an external drive once i have the mods stable. I have windows 7 64bit now
. I have installed Oblivion but SI part in on the external drive. There is a tool out there that strips out all files added by mods, leaving me with Oblivion -SI to copy over the new Oblivion install on my windows 7 rig.