WinXp connecting backwards

I’m using a WinXp notebook and I’m trying to connect back to Window 2K.There is a authentication box pop-up.There is no password set on the Win2k as well.Same happen to my Windows98 WS where an aunthentication box pop-up when trying to connect to a WinXp notebook.My network is very simple with a few workstation and no DNS and WINS server around