Tried that. Still nothing. I did the command line stuff in that link, no help. I still have the Windows CD (the guy in that link doesn't), and I can boot from the CD, but it prompts me to "Install Now" and my only options in there are Upgrade or Custom (i.e. fresh install).
There is a "Repair your computer" option, but that just gives me a message saying:
This version of System Recovery Options is not compatible with the version of Windows you are trying to repair. Try using a recovery disc that is compatible with this version of Windows.
...but it's the same disc I used to install it. It's an OEM version, so I assume this is my problem, but I'm not sure. I'm not opposed to just buying another license, but I just wanted to make sure I had no other choice.