Shellcode

["Pedro E" <pedro () enzona com>]

Hi All! 

I'm looking from some orientation to write a specific shellcode,
I think that is a simple problem but I can find a solution.
I have to execute a simple shellcode using hard coded addresses with a 
msgbox: 

LibraryReturn:
      pop ecx                         ;get the library string
      mov [ecx + 10], dl              ;MY PROBLEM is this line I don't have 
the right permissions to modify the NULL value and finish the string
      mov ebx, 0x79470221             ;LoadLibraryA(libraryname);
      push ecx                        ;beginning of user32.dll
      call ebx                        ;eax will hold the module handle 

      jmp short FunctionName 

xxx
..
.. 

GetLibrary:
      call LibraryReturn
      db 'user32.dllN' 

There is other way to set the null value? I can't modify the memory when i 
execute the shellcode.
Anybody have any tip to bypase this problem?
Thanks a lot.
Pedro Ezcueti.