you gotta make sure "File and Printer Sharing" is enabled on both machines. Then make sure you're firewall is allowing NetBIOS/NetBui since this is the protocol that Microsoft uses to communicate. A lot of firewalls deny this so that hackers can't see you're shared folders (this is really easy to do, especially if you don't have them password protected).

I had this problem, and that was my solution (to fix the firewall)