format PE GUI 4.0
entry start

include 'win32a.inc'

section '.text' code readable executable

  start:

        invoke  GetModuleHandle,0
        mov     [h_exe],eax
        invoke  sprintf,message,fmt,[var]
        invoke  MessageBox,HWND_DESKTOP,message,caption,MB_OK
  exit:
        invoke  ExitProcess,0

section '.data' readable writable

  caption db 'MessageBox Caption!',0
  fmt db 'Variable value is: %e',0
  var dd 100.0

section '.bss' readable writeable

  h_exe dd ?
  message rb 100h

section '.idata' import data readable writeable

  library kernel,'KERNEL32.DLL',\
          user,'USER32.DLL',\
          msvcrt,'MSVCRT.DLL'

  import kernel,\
         GetModuleHandle,'GetModuleHandleA',\
         ExitProcess,'ExitProcess'

  import user,\
         MessageBox,'MessageBoxA'

  import msvcrt,\
         sprintf,'sprintf'    

That function needs a double instead of simple float. A quick solution is to use win32ax.inc, use "var dq 100.0" and "invoke sprintf,message,fmt, double [var]"