I have samba running on Ubuntu box. I can see the windows shared folders from the ubuntu box when I browse Windows Vista Ultimate X64. The Windows 7 notebook doesn't even show up. All three are in the same workgroup. All three have shared folders and drives. I cannot see the Ubuntu baox from Windows 7 notebook. Running Win7 Ult X64.
How about typing two of the goofy forward slashes followed by the Ubuntu box's IP address in the "Run" dialog in Windows? For example, if your Ubuntu machine has an IP address of 192.168.1.100, type "\\192.168.1.100" into the Run dialog box. If you don't know the IP address, go to the Ubuntu box, open up a gnome-terminal and then type in "/sbin/ifconfig" and look for whatever connection isn't loopback (lo.) That might be eth0 or wlan0 or something of that sort. The IP address will be below that.
Sometimes Windows doesn't properly detect shared folders on the network, even if they are hosted on another native Windows computer. But forcing the machine to try to look for shared folders with the Run command nearly always does the trick.