Sharing Windows folders for access by Ubuntu Linux is fairly straightforward using Samba (SMB) but going in the opposite direction had me stumped.
From Windows, I would go into My Network Places ~ Entire Network ~ Microsoft Windows Network ~ Workgroup and I could see the Linux machine. Then I would double-click on it and enter my Linux name and password, but it would not connect.
The solution is that you need to add a Samba user, and you can't do that from the Linux GUI. You have to open a Terminal window and add the Samba user like this, where username is your Linux user name:
sudo smbpasswd -a username
after which you are prompted for the password, which is your Linux password.
At that point, you can access your Linux shares from Windows just like you would any other network share.