include '%fasminc%/win32ax.inc'
WM_SYSCOMMAND = 112h
SC_SCREENSAVE = 0f140h
.code
start:
invoke GetDesktopWindow
invoke PostMessage,eax,WM_SYSCOMMAND,SC_SCREENSAVE,0
invoke ExitProcess,0
.end start  
    

format PE console

include '%include%\win32a.inc'

WM_SYSCOMMAND = 112h
SC_SCREENSAVE = 0f140h

section '.code' readable executable

  entry start

start:
        invoke GetDesktopWindow
        invoke PostMessage,eax,WM_SYSCOMMAND,SC_SCREENSAVE,0
        invoke ExitProcess,0

section '.idata' import data readable writeable

  library kernel, 'kernel32.dll', \
          user, 'user32.dll'
  import kernel, \
         ExitProcess, 'ExitProcess'
  import user,\
         PostMessage,'PostMessageA',\
         GetDesktopWindow, 'GetDesktopWindow'    

I know ,what I'll say can be little strange but your code dryobates dosn't work a well as mine.

is it possible that windows or any programm lock the messages?

btw : let you upgrade your FASM version to latest 1.51

so I think you changed %fasminc% to %include% to support older fasm's sources.(right?)

Windows does not start the screen saver if any of the following conditions exist:

- The active application is not a Windows-based application.
- A computer-based training (CBT) window is present.
- The active application receives the WM_SYSCOMMAND message with the wParam parameter set to the SC_SCREENSAVE value, but it does not pass the message to the DefWindowProc function.