I found a huge source for MSE definition updates tonight.

Stumbled onto it as I had the idea of searching for all URLs containing the search string below once I found a complete URL for one of those 1,040 files that worked.

https://web.archive.org/web/*/https://definitionupdates.microsoft.com/download/DefinitionUpdates/*

Breakdown:

808 MPAM-FE.EXE captured 04/18/2013 to 02/04/2023
232 MPAM-FEX64.EXE captured 07/13/2014 to 07/24/2022

You can trust these since they're straight from Microsoft via archive.org and best part you can search the version numbers in many cases before you download them.

(More than I can say than for where you can get the MSE installers from...haven't found an MS archived source for those yet.)

Some of these may be duplicates (obviously there weren't really 1,040 unique versions), but still.

Now...

I found two versions of 64-bit MSE that will successfully install under XP Pro x64, but curiously the newer version (4.5.216.0) has to be installed AFTER the older one (4.4.304.0) and that seems to be the very last installable version (that doesn't throw up any error).

Source for older MSE versions (there's lots on this page both 64-bit and 32-bit):  https://www.filehorse.com/download-security-essentials/

That older version is the same one that also installs under XP Pro x86, except 32-bit.

The problem is all the definition updates I tried up until this find did not work.  I struggled for hours a couple days ago. It's possible I didn't try enough as before this find I only had a couple to work with.

Steve

I've exhausted this much further and came to the conclusion that no matter what the facts are about being able to install Microsoft Security Essentials on Windows XP Pro x64 SP2, that Real-Time Protection cannot be turned on. That is the entire problem. It cannot be done.

I even found a Real-Time Protection key in the registry into which an entry can be made to enable it, but it had no effect.