Try this: set Windows Update to "never check for updates." If you have disabled a Windows service, re-enable it.
You can help wiki How by improving the current introduction, or writing a new one to match the format described in the Writer's Guide.
Please see some free third party tools, such as Avast Antivirus, Bit Defender Free or AVG. Microsoft Security Essentials uses Windows Update to download and install new definitions once a day.
If updates fail constantly, follow instructions in the Reinstall Windows Update article.
Assuming this works, it might be a good time to try the solution for Windows updates suggested in your other thread.
Attention, Windows XP and Vista users: Microsoft Security Essentials turns itself off as your operating systems are not supported anymore.