I have received substantial help from Nasdaq on the Spyware malware forums: http://www.spywareinfoforum.com/topic/136678-ms-vista;-will-not-update-windows/
Ultimately, Nasdaq says that I have no malware issues and that I should reach out to this forum for subsequent assistance.
OS: Windows Vista Home Premium, 32 bit
Older PC, maybe 9 years or so. About 2 months ago, it stopped installing MS updates. It will download the updates, but will not install them. I cannot summon the updates by checking for Windows Updates. The request says "Checking for updates" and never goes any further. I leave the PC on overnight and it will automatically retrieve the updates, but they will not install. All the update failures are labeled with Error Code 80070020.
I have already tried clearing out the update cache; no change. I have tried to download the MS Update fixit tool. My PC doesn't know what program to use to read a Diagcab file. Neither do I. Leading up to the inability to run the Windows update, the PC was running very slow. I ran ComboFix, RKill + Malwarebytes, and I also used Wise registry cleaner. It is entirely possible that I have deleted important Windows Update registry keys; not sure.
I am reasonably familiar with moving in and out of programs and CMD prompts, etc. What next?