Vista - Strange behaviour!

I have 3 computers:

(A) XP Pro SP3
(B) XP Pro SP3
(C) Vista HP SP1


Connecting (C) with (A) by Ethernet cable everything works, while with (B)
there's something wrong, that is:

- (A) can see/ping/access shared folders and use shared Internet connection
of (C), and vice versa: this is good

- (B) can access shared folders of (C) but if I try vice versa, that is (C)
accesses shared folders of (B), than an access window appears asking
username+password; this is very strange because

1) the user of (B) hasn't password (default); I've tried to use
admin+blankpassword or PCname\admin+blankpassoword or
PCname\myuser+blankpassword and so on.. but auth. fails; what's the correct
form of the username and password I have to enter into this mask? isn't it
stange it asks to me to login? (A) doesn't ask anything and it's the same
(XP Pro SP3)

2) maybe due to issue 1 (C) can't use shared Internet connection of (B); is
it possible?


I really don't know how to solve this problem. Thanks for any advice in
advance.


Massimo

No update. Always Vista can't see XP Pro..

Massimo