debuging

hello,=0D
  I am a newbie in win32 software hacking.=0D
  when i have a open source software,i use the gdb to debug the software,=
but the most of win32 app, is not a open source,why i know the functions?=
the operations?what tools i will use to this?=0D
  For example is simpliest search buffer overflows in open code,why to do=
 this in non open source software?=0D
=0D
thank=B4s