Windows Defender doesn't start at startup

Even though I've set Windows Defender to startup at startup, it doesn't.
I've even got the option set in its options to always display the Windows
Defender icon in the system tray. And the Windows Defender service is
automatic and always started. But when I login to my PC, it doesn't startup
in the system tray, nor is the MSASCui.exe service running.

And it never notifies me of changes I make to the system even though I have
all the options checked in its options. It only notifies me if I start
Windows Defender from the Start menu. But it never starts up at startup
anymore.

In MSCONFIG, it's checked as well.

What can I do to make it startup at startup instead of me having to launch
it manually on every boot. I don't want to put it in my Startup folder
either. I want it to work on its own like it's supposed to. And I can't find
a way of reinstalling it in Windows Vista either.