The Icon in the system tray isn't called defender, it's a grey shield, and if you hover over it, it should say something like "System Status: Protected".
"On my old Windows 7 PC, I could easily see how much of the hard disc was being used at any time, by bring up a simple pie chart, from "My Computer"
How can I do this in Windows 10 please ?""
Click Start, from the right-hand menu, click This PC
This will show your drives as bar-charts rather than pie charts.
If you right-click a drive, and choose properties, it will show a circular bar chart.