🚀 go-pugleaf

I have 2 small programs that potentially could interfere with one another:

The first is CustomSetup.exe: It installs AutoRun.exe and MainApp.exe on 
removable media. If the media is not inserted, a message box pops up telling 
the user that the media must be inserted in order to install with a 
MB_RETRYCANCEL option.

The second is AutoRun.exe: It is placed on the removable media by 
CustomSetup.exe, and prompts to start MainApp.exe with CreateProcess 
whenever the media is inserted.

The Issue:
A Techs installs the program by running CustomSetup.exe. CustomSetup.exe 
(first detects that MainApp.exe is not already installed, then) detects that 
there is no removable media, and informs the Tech that the media needs to be 
inserted. The Tech inserts the media, which just happens to have AutoRun.exe 
and MainApp.exe already on it.

The Problem:
If AutoRun.exe can detect that CustomSetup.exe is running, it can close 
CustomSetup.exe ... or itself.

I'd like to use EnumWindows and GetWindowThreadProcessId within AutoRun.exe 
to find and close CustomSetup.exe (using WM_CLOSE), but I don't know how to 
locate it.. The closest functions I have found are GetWindowText and 
GetClassName, but it appears that neither is suitable since both are small 
"WinMain Only" programs (neither has a class or a window).

How can I find the CustomSetup.exe process? Any ideas?