Stack overflow

Hi,

When I was trying to understand the complexity of the stack operations I found a discussion on "stack overflow". I became very curious and tried to make my own stack overflow, using a little "shellcode" made by my self:

Code:

; buffer overflow test

format PE GUI 4.0
include '%fasminc%\win32a.inc'

section '.code' readable executable

entry $

  push 0
      push title
  push caption
        push 0
      call [MessageBox]

       call FillBuffer

 push 0
      push title
  push caption
        push 0
      call [MessageBox]

       push 0
      call [ExitProcess]

      proc FillBuffer

         push ebp
            mov ebp, esp
                sub esp, 72
         push shellcode
              push esp
            call [lstrcpy]
              leave
               ret
 endp

    title db 'BOF test', 0
    caption db 'Testing stack overflow', 0

    shellcode:
  nop
 nop
 nop
 nop
 xor eax, eax
        push eax
    push eax
    push eax
    push eax
    mov eax, 7E3A07EAh ;MessageBox (WinXP sp3)
  call eax
;ERROR HERE
     mov eax, 7C81CB12h ;ExitProcess (WinXP sp3)
 call eax

        db 'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA', 70h, 0FFh, 06h, 00h ;0006FFA4

section '.idata' readable import

        library kernel32, 'KERNEL32.DLL', user32, 'USER32.DLL'
  import user32, MessageBox, 'MessageBoxA'
  import kernel32, lstrcpy, 'lstrcpy', ExitProcess, 'ExitProcess'

The problem is:

Debugging this program on OllyDbg, the program crashes when returning from call eax (MessageBox WinXP sp3). Please help me to understand why it crashes. Maybe MessageBox function destroys the "shellcode" but I didn't see that.

Thankyou.