Or if Windows Defender won’t let you get the latest definitions.
Check the file C:\Windows\WindowsUpdate
Scan for the following error
“Client has an invalid PID”
If so then perform the following.
*Open an elevated command prompt. (Go to Start -> Programs -> Accessories,
right-click on Command Prompt, and choose “Run as administrator”.)
*In the command prompt window, run the following two commands:
net stop wuauserv
rmdir /s C:\Windows\SoftwareDistribution\DataStore
Don’t close the command prompt window yet.
*Rename the C:\Windows\WindowsUpdate.log file to some other name. (This
isn’t actually necessary, but will make it much easier to distinguish the
post-reset activity from the pre-reset activity.)
*Run the Registry Editor. (Go to Start -> Run, type “regedit” and press
Enter.) Warning: Incorrect registry changes can damage your Windows
installation and require reinstalling Windows or other software; be very
careful when following these steps.
*In the Registry Editor tree, find
Right-click on WindowsUpdate in the tree and choose Delete.
*Back in the command prompt window, run this command:
net start wuauserv
*You can now close the command prompt window and the Registry Editor window.
*Now run the Windows Update control panel