Remember localhost always means "this machine". Same with the IP address 127.0.0.1. If you already knew that please disregard this paragraph.
Make sure both "Personal File Sharing" and "Windows Sharing" are enabled in the Sharing system preference pane on the Mac. Make sure the firewalls on both machines are allowing Windows file sharing, or make sure they are both off. Make sure both machines can ping each other.
Make sure both machines are in the same workgroup. On Windows: Right-click My Computer and click the Network Identification tab. On Mac: Open /Applications/Utilities/Directory Access.app, click the lock icon if the list is grayed out, make sure SMB is enabled, and double-click it to get at the workgroup setting. Neither OS is case-sensitive with the workgroup, but all caps is the custom.
Try "\\ip address\share" instead of "\\host name\share".
If all that doesn't work, make it so that there is a user account on each machine with identical user names and passwords, make sure that user has something shared from the Mac, log in to Windows with the same user and try again. Might help to reboot both machines after setting them up properly and immediately log in to both with that user.
It would be sweet if MS threw Rendezvous into Longhorn for printer and file sharing discovery. Not to mention incredibly easy. (Rendezvous is dirt-simple to implement on any OS.) Would also be nice if both Apple and MS would make SMB/CIFS authentication dialog boxes always have separate fields for name, password, and domain/workgroup. Lots of things would be nice
