Samba was working just fine with Fedora Core 1. I installed Fedora Core 2 and now I am unable to log onto the Windows machines. I can see them, but when I try to open one, whereas it used to ask me for my username and password, now I just get an error message saying I don't have permission to view this domain. I am using the same username and password as ever. I can see localhost from the windows machine also, but when I try to open it on the Windows machine I get an error: inaccessible, duplicate name exists on the network. I am using Win2K. Samba is set to user and no guest.

You need to change the name from something besides localhost. The reason it is a duplicate name is due to the fact that localhost is assigned to 127.0.0.1 i.e. the lo interface on every TCP/IP configured PC. Add a line to the global section of the /etc/sambasmb.conf file

netbios name = The_PC_Name (Change to fit your needs)
Be sure to restart samba

Did you also add a smbpasswd for your user?