Follow the guide to properly remove the application then re-install it using the official download.
Alternatively, go to HKEYLOCALMACHINE\Software\Microsoft\CurrentVersion\Run and look for MSC which is a string value. If it does not exist, create it and add the following parameter: "C:\Program Files\Microsoft Security Client\msseces.exe" -hide -runkey. If the path is different (x64 operating system), replace it between the quotes. Restart it and the application will have its icon in System Tray.