Are you using Avast 6.0 recently updated?
Id it’s true, you must to revert to Avast 5.1.889 because v6 have an issue with smb folder with Wii and Xbox

You can read more about in these links:
http://www.wiimc.org/forum/viewtopic.php?f=4&t=1188
http://forum.avast.com/index.php?topic=72798.0

I just did this, adding a shared folder to wii MC in windows 7. I right clicked the folder, properties, sharing tab, I hit the share button and added a new person by typing in Everyone and than gave it read permissions and then in Wii MC i filled in the folder name network ip and left Username and Password blank and it worked. Maybe this will help some people.

If this doesn’t work you might need to add the User “Everyone” by hitting the advanced sharing button in the properties-sharing tab. Then go permissions and added a new permission for “Everyone” by hitting the add button and than typing Everyone in the enter object names and gave it all permissions applying it to folder, sub folders and files.

This poses a security risk if your network is shared by many.

-You may also have to disable network password protection.
-There’s probably a more secure way to do it like adding a new User Account under user account control and than adding this user instead of Everyone than in WiiMC this username and password. But what i did worked and i’m satisfied.