I had this issue as well, the solution is this registry key;
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\LanmanWorkstation\Parameters
Change the value of AllowInsecureGuestAuth from 0 to 1.
Its the kind of setting that a work computer will have configured via group policy as it is quite insecure. My advice would be to change the key back to 0 again when you are done.
EDIT: Just for completeness I should describe what it does. This setting allows (or denies) the use of a network share when no password is required to connect. Guest authentication.
EDIT2: Just realised you already found the solution. So, as you were 