microsoft.public.windowsxp.security_admin

Hi ! I experiencing a quiet strange problem.
I installed a xp pro, not connected to any domain. but I still can access to
the DC by \\server\c$ after being asked for user and password.
But when I try to connect to a regular xp workstation, or vista workstationt
I get
" logon failure: the user has not been granted the requested logon type
at this computer."
"use simple sharing" is disabled on both sides.
But, I if do a network map, specifying the user and password, it works.
So why doesn't it asks for user/password after typing "\\my.pc\c$" ?
It worked fine before the windows 2000-> XP upgrade.
Any ideas ?
thanks !