vista pc won't access network shared folders

We have a network domain, all XP pro compters, though the domain controller
is NT. I have successfully joined this vista pc, and can 'see' all of the
computers and servers on the domain, BUT cannot access any of the shared
folders.
Can't map to them which is what we usually do, can't browse to them, it asks
for a username and password but will not authenticate using good credentials,
actually will not even authenticate using the server credentials.
I have exhaustively checked the network settings but do not know where else
to look.
I am using our standard network dns, wins, gateway and IP in TCP/ IPV4
We will soon be moving our 300 pc's to Vista, and this is our test pc, so I
need to get this one right before we proceed.
I was wondering if it has to do with active directory? We are planning to
move to active directory soon as well.