Vista / W2K File Sharing

I have a new Vista machine that is a new member of a W2K peer network. Both Vista and W2K machines see each others shared drives. From any W2K machine, I can browse any shared drive on the Vista machine without entering a user name and password. However, From the Vista machine, I can only browse shared drives on the W2K machines after entering a valid user name and password (of the W2K machine). I tried enabling the guest account on the W2K machine but no joy. What am I missing?

John