laptop is sluggish to the point of being unusable

In short, my Dell Latitude E4310 laptop is sluggish to the point of being unusable. I am also unable to keep the Windows Defender service running. Last week I had a blue screen error (code c000021a), presumably resulting from an improperly applied Windows update. I was able to return to an earlier instance of Windows and successfully apply the patches. Here's what I've checked / done...