; http://www.betamaster.us/blog/?p=757

format PE GUI 4.0
include 'D:\FASM\INCLUDE\WIN32AX.INC'

entry start
;================== code =====================
section '.code' code readable executable
;=============================================

proc start

        invoke FindWindow, NULL, WindowTitle ; Find the window titled ‘Notepad’
        test eax,eax ; Test whether a window with that title was found or not
             jnz .ju1 ; Don’t jump = The window was not found
             invoke MessageBox,0, message1, _caption, MB_OK ; – Display an error message (Window not found)
             jmp .exit ; – Exit the application
             .ju1: ; Jumped = The window was found
        invoke GetWindowThreadProcessId, eax, ProcID ; Get the ProcessID via the window handle
        invoke OpenProcess, 0x1F0FFF, FALSE, [ProcID] ; Open the process using PROCESS_ALL_ACCESS (0x1F0FFF) and get a handle
        mov dword[ProcHandle],eax ; Save the handle

        ; VirtualAllocEx: Reserves/commits a region of memory within the virtual address space of out target process
        ; We should do this in order to avoid potential access violations (which might cause crashes)
        invoke VirtualAllocEx,dword [ProcHandle], startAddress, _patch.size, MEM_COMMIT, PAGE_READWRITE
        cmp eax, 0 ; EAX == 0 : Failed to reserve the memory region
        jnz .cont ; EAX != 0 : Continue with further steps
        invoke MessageBox,0, message4, _caption, MB_OK ; Display an error: VirtualAllocEx failed to reserve the memory region
        jmp .exit ; Exit the application
        .cont:
        invoke WriteProcessMemory, dword[ProcHandle], startAddress, patchBytes, _patch.size, patchResult
        cmp [patchResult],_patch.size ; Compare the number of patched bytes with the length of our new bytes
        je .ju2 ; Don’t jump = Failed to patch the target
        invoke MessageBox,0, message3, _caption, MB_OK ; – Display an error message (An error occured)
        jmp .exit ; – Exit the application
        .ju2: ; Jumped = Target patched successfully.
        invoke MessageBox,0, message2, _caption, MB_OK ; Display: The target has been patched successfully
        .exit: ; Jumper: Here we’re going to exit our application
        invoke ExitProcess, 0 ; ExitProcess

endp

;=================== data ====================
section '.data' data readable writeable
;=============================================

WindowTitle db 'Unbenannt - Editor', 0
ProcID dd ?
ProcHandle dd ?


_caption db 'Information', 0
message1 db 'Unable to find the window', 0
message2 db 'Patched successfully',0
message3 db 'Patching: An error occured',0
message4 db 'VirtualAllocEx failed',0

startAddress dd 0x00401090 ; The memory address we’re starting to write from
patchBytes   db 0x2F, 0x66 ; These bytes will be written into the memory of our target executable
_patch.size = $ - patchBytes  ; Holds the number of bytes we’re going to write
patchResult dd ? ; Holds the number of successfully written bytes

;=============================================
section '.idata' import data readable
;=============================================

library kernel32,'KERNEL32.DLL',\
user32,'USER32.DLL'

import kernel32,\
       ExitProcess,'ExitProcess',\
       OpenProcess,'OpenProcess',\
       VirtualAllocEx, 'VirtualAllocEx',\
       WriteProcessMemory,'WriteProcessMemory'

import user32,\
       FindWindow,'FindWindowA',\
       GetWindowThreadProcessId,'GetWindowThreadProcessId',\
       MessageBox,'MessageBoxA'     

I found a well explained example about WriteProcessMemory but the coder did put some bugs inside the code. Some errors i did fix but is not enough, the byte patching does not work for me