I had the exact same problem about a week ago - tried permissions and everything. This website helped me out:

This is in some cases caused by a registry setting named RestrictAnonymous. Go to the computer which you cannot access, start a registry editor and change the following registry value.



HKEY_LOCAL_MACHINE
\SYSTEM
\CurrentControlSet
\Control
\Lsa
Value name: RestrictAnonymous
Value type: DWORD



If the value is 1 or even 2, change it to 0, reboot and retest. If the problem is solved, leave the value at zero. If not, you can change it back if you like.