You might not have permission to use this network resource

Source
\\(COMPUTER NAME)\shareddocs is not accessible
Solution:

Turn off any firewalls for debugging. If the firewall is the problem, you'll have to configure it to allow access to "trusted zone" addresses. Note that some firewalls must be completely uninstalled to stop them from affecting your networking.
PING each computer by IP address, and if successful, PING by name. You can obtain the IP address of a computer by opening a command prompt (DOS window) and typing IPCONFIG. This should work for any Windows version.
All computers should be in the same workgroup for computer browsing to function properly. File & Print Sharing has to be enabled on any computer you wish to share files or printers from. You also need to actually share the resource in question from My Computer, right click on the drive/printer/folder, and select sharing.
If you encounter difficulties accessing computers that are visible in Network Neighborhood, make sure the computer being accessed has an account with the same name/password as the system connecting to it uses to login.
While the default NetBIOS setting is correct for normal network configurations, it's possible for it to be altered, and it costs nothing to make sure it's correct. NETBIOS over TCP/IP must be enabled for normal network browsing.
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Fuente Este problema se produce si la entrada RestrictAnonymous del Registro está establecida en 1 en el equipo basado en Windows XP.

1. Haga clic en Inicio y en Ejecutar, escriba regedit en el cuadro Abrir y haga clic en Aceptar.
2. Busque la siguiente subclave del Registro y haga doble clic en ella:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa
3. En el panel derecho, haga doble clic en RestrictAnonymous.
4. Asegúrese de que el valor del cuadro Información del valor es 0 y haga clic en Aceptar.
5. Cierre el Editor del Registro.
6. Reinicie el equipo.

Locate the subkey in Regedit, and if the value of 'restrictanonymous' was 0, then try to modify the value of another subkey im same location named 'LimitBlankPasswordUse' and set it to 1

Source Please be aware that using the restrictanonymous=0 setting will allow anonymous users to see network shares, users and security permissions. Do not do this on a domain! I know this is what you are trying to do for windows networking but think about just adding a user to the machine in question so that authentication can take place normally. That way just anyone who gets into your network can't access the server/workstation.
IE: If Joe with password 1234 wants access to a computer on the network, add Joe with pass 1234 to that machine, set his rights and what share he can access, printer rights, whatever. He can then access using the normal computer he's logged into. ---icmpechome
RestrictAnonymous
HKLM\SYSTEM\CurrentControlSet\Control\Lsa
Data type Range Default value REG_DWORD
0 | 1 | 2

Description
Restricts anonymous users from displaying lists of users and from viewing security permissions.
Value Meaning
0
Disabled. Anonymous users are not restricted.
1
Enabled. Users who log on anonymously (also known as null session connections) cannot display lists of domain user names, nor share names. Also, these users cannot view security permissions, and they cannot use all of the features of Windows Explorer, Local Users and Groups, and other programs that enumerate users or shares.
2
Anonymous users have no access without explicit anonymous permissions.
This entry does not exist in the registry by default. You can add it by using the registry editor Regedit.exe.
Note

• Do not set the value of this entry to 2 in mixed-mode environments. Only consider setting it to 2 in environments running only Windows Server 2003, and only after verifying that appropriate service levels and program function are maintained.

Caution
Pre-defined "High Secure" security templates set the value of this entry to 2. Use caution when using these templates.