Here is what I recommend - create a shortcut that uses wscript to execute a VBS. I use this all the time to make authentic Windows dialogs like the below.
- In File Explorer, right-click - select New and click "New shortcut". Then type the following "C:\Windows\System32\wscript.exe "error.vbs"" where error is the name of your VBS script
- Create a VBS script with the name you chose that looks like this
x=msgbox("Windows Defender has detected one or more viruses infecting this machine. To protect the integrity of your operating system and keep your files safe, please run a complete scan from Windows Defender to purge your system of any leftover malware.", 0+16, "Windows Defender Has Discovered Malware")
The 0+16 is the Button+Icon code. Here are the number codes for the icons you can use.
Button =
0 - OK
1 - OK and Cancel
2 - Abort, Retry and Ignore
3 - Yes, No and Cancel
4 - Yes and No
5 - Retry and Cancel
Icon =
0 - No Icon
16 - Critical Icon
32 - Question Icon
48 - Warning Icon
64 - Info Icon
You may also refer to them by name.
- The VBS and the shortcut NEED to be in the same folder.
- Now, go to the properties of the shortcut you created. Change the icon to an authentic (realistic) Windows error or message icon. Realistically, it should correspond to the meaning of the number you chose for the Icon.
In the end, you get something like this:

(Yes, I made up the message)