Is this possible to replace windows xp logon ui (i mean gina) with a win32 application? I don't talk a about writing a dll and replace it with windows original gina dll, I mean a proper interactive win32 application like notepad.exe
Many software vendors override the Logon UI to run programs such as Fingerprint or Facial Recognition scanning software. So yes you can override the default Windows Login UI even up to Windows 7.
However, it is only available to perform login operations. You won't have access to certain system resources like mapped drives. So if the intent was to be able to do certain things you would be stuck. Again, If you are trying to run a kiosk style environment you could perform an auto-login and then have the program automatically open once Windows has loaded.
Any particular reason you are trying to do this with XP? Its support from Microsoft runs out pretty soon.