Scheduling silent hourly Windows Defender definition updates using Task Scheduler on Windows 8

Use a VBS file instead of a CMD file and schedule it as usual with your Task Scheduler.

VBScript's run method can open other programs in a hidden window via its second argument (, 0). The tricky part was the escaping together with the argument -SignatureUpdate

set objShell = createobject("wscript.shell")  
objShell.Run("""C:\Program Files\Windows Defender\MpCmdRun.exe"" ""-SignatureUpdate""") , 0

Now you won't see any window during Windows Defender update. Only a task manager process is visible:

---

Other possible settings for intWindowStyle:

0 = Hide the window and activate another window.
1 = Activate and display the window. (restore size and position).
2 = Activate & minimize.
3 = Activate & maximize.
4 = Restore. The active window remains active.
5 = Activate & Restore.
6 = Minimize & activate the next top-level window in the Z order.
7 = Minimize. The active window remains active.
8 = Display the window in its current state. The active window remains active.
9 = Restore & Activate. Specify this flag when restoring a minimized window.
10 = Sets the show-state based on the state of the program that started the application.

This works pretty well but opens up a cmd window every hour and I want to run it silently in the background.

I always use task scheduler on MSE and in W8 on Defender, with the parameters you also state. To prevent the cmd window to open you have to change the user account, on the general tab in the task scheduler, to SYSTEM and check the box high priority.

Now it should work smoothly!

1. Instead of using %ProgramFiles%\Windows Defender\MpCmdRun.exe -SignatureUpdate, try
   %ProgramFiles%\Windows Defender\MSASCui.exe -Update instead.

2. If option 1 does not work, you can use Hidden Start to hide the cmd window:

Console applications and batch files are regularly run at Windows startup or in a schedule. The main inconvenience of this is that each application opens a console window that flickers on the screen. Hidden Start (or Hstart) is a lightweight command line utility that allows you to run console applications and batch files without any window in the background, handle UAC privilege elevation under Windows 7 and Vista, start multiple commands in parallel or synchronously, and much more.