Funnily enough, I just fixed it myself. I went through the custom WUA.EXE installer, comparing the file sizes to what was actually installed, and was surprised that roughly half the files were the wrong versions. So I copied these .dll's, .exe's, and .mui's to their proper locations. Winhttp.dll was a little tricky. At that point, Windows Update skipped past the error, but cited that BITS/Winhttp wasn't installed. I tried installing it through Windows Update, and again it failed. So I reinstalled it manually. After a reboot, Windows Update is now working normally, and I was able to download and install a small Security Update. So problem solved. Thanks everybody, and thanks ppgrainbow!