I'm at my wits end. I'm using a BIOS mod with ASUS SLIC 2.1. I have a few other similar computers, but one of them is no longer genuine. I used a regular DVD to install, and then installed the cert and key manually. Now it's not working. I'm going to try MBR Regenerator and then DAZ. I don't know why I'd need them, since several other machines are running fine without either. Although I don't think KB971033 was installed on those machines, I did find it on the malfunctioning one.
Welp, please close this thread. I still have absolutely no idea what went wrong, but MBR Regenerator, DAZ, then WAT seemed to fix it. Thanks for being such a genuine resource.