You can "Repair Windows" from the installation disc, but that hasn't worked for me. What I'm trying to do is do an upgrade install (or repair install) from my bootable USB with Windows 7 Ultimate on it. This seems to be impossible when booted from that USB, which means I don't have many options other than a clean install since I can't boot Windows from my HDD. I've come across a few places online where people make it sound like it might be possible to "trick" the installation into thinking the computer is booted from the HDD, allowing a repair install from the USB, but so far I haven't found anything conclusive.