One XP computer on network cannot access Windows 7 computer

I've got a total of six computers on my network - 2 running Windows 7 and 4 running XP. With the exception of one computer, all of the machines see each other on the network and can share files. One XP machine does not see the shared folders of my new Win 7 machine but does list it as a "workgroup" computer. When I try to access the computer itself I am prompted for a password. Passwords are...