If Windows Defender is "off" should MsMpEng even be running?

Question

Note: system is Windows 7

Lately my computer's been grinding to a halt for a couple minutes at a time throughout the day for no apparent reason and with no obvious pattern. All I could see in the Task Manager is that CPU usage spiked, though it didn't seem to be caused by anything visible in the process list.

Eventually, in Resource Monitor I found MsMpEng.exe (not visible in TM, at least not with that name), which is evidently a core process for Windows Defender, also doing an enormous amount of grinding on the disk among other things right when one of these was happening. So that's the current suspect. I'm waiting for another one to confirm/deny.

Besides that, though:
When I went to check if there were any settings I could tweak, it turns out that Windows Defender is "turned off" (that's what the dialog says). So should this process even be running in the first place?

The dialog also has a link to turn the program on that presumably should go to some options or something, but that just spins the activity cursor forever, and I still haven't found how to access/uninstall it otherwise, so it's possible the entire thing is just kind of broken at the moment. (It doesn't show up in the dialog to toggle Windows features, for example.)

Answer 1

If Windows Defender is “off” should MsMpEng even be running?

Only and only if you have Microsoft Security Essentials or System Center Endpoint Protection installed.

Otherwise, no.