Windows Defender/Update Error 0x800b0001: Unknown trust provider

Hi Guys,

I'm getting the following pop-up from Windows Defender when I start the computer. The same message pops up when I try to start the service manually:

"Windows could not start the Windows Defender service on Local Computer.

Error 0x800b0001: Unknown trust provider."

I've also noticed the Windows Update is unable to search for new updates, giving an Error Code 800B0001.

I've done some searching around the web and nothing I've done seems to work. What exactly is this error and does anyone have any suggestions as to how to fix it?

Thanks.