I ran into an issue where Windows Defender couldn't download updates, when I would try, I would receive the error:

"The Program can't download definition updates. Error Found 0x80240022"

This was occurring on a spyware ridden machine, and I was worried that it may be a piece of malware that was preventing the download. Maybe if I downloaded and installed the definitions manually, I would be able to run the scan.

After installing the definitions manually, Windows Defender was able to update itself. I'm not sure why, but that was the fix for this particular issue.

If you ever have a need to download Windows Defender definitions manually, the following link always points to the current definitions for the 32 bit versions of Windows:

http://go.microsoft.com/fwlink/?linkid=70631

If you are running an x64 version of Windows, this is the link:

http://go.microsoft.com/fwlink/?linkid=70632

Finally, if you are running an Itanium version of Windows, this is the link:

http://go.microsoft.com/fwlink/?linkid=70633

I found out about these links in Windows KB article 923159.

Reference:

How to manually download the latest definition updates for Windows Defender - Microsoft