For folder sharing to work in Windows, there are at least five things that need to happen:

1. The file sharing feature must be enabled on the computer that's sharing the folder.
2. The folder being shared must have its "Sharing Permissions" set properly.
3. The folder being shared must have its NTFS disk/file permissions set properly.
4. The Windows Firewall must have the correct openings for folder sharing. (This is done for you by default when you enable the sharing feature.)
5. The "Server" service must be running on the computer that's sharing the folder. (This is default on all systems.)

There may be other things that need to be enabled. I've had situations like yours, when I still get errors like you describe even when I've met all of the above conditions. Usually, in times like that, I give up and sneakernet the files.