This is probably due to your network group. Try changing the network location of your computer to either Home or Work!
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The computer is running Windows 7 or Windows Vista and the Link Layer Topology Discovery (LLTD) protocol is disabled on the network adapter. Windows uses the LLTD protocol to map computers and devices on a network. To enable LLTD, follow these steps:
Open Network Connections by clicking the Start button Picture of the Start button, and then clicking Control Panel. In the search box, type adapter, and then, under Network and Sharing Center, click View network connections.
Right-click the network connection, and then click Properties.
Select the Link-Layer Topology Discovery Mapper I/O Driver and Link-Layer Topology Discovery Responder check boxes, and then click OK.
The computer is running Windows 7 or Windows Vista and network discovery is turned off. To turn on network discovery, follow these steps:
Open Advanced sharing settings by clicking the Start button Picture of the Start button, and then clicking Control Panel. In the search box, type network, click Network and Sharing Center, and then, in the left pane, click Change advanced sharing settings.
Click the chevron Picture of the chevron icon to expand the Home or Work network profile.
Click Turn on network discovery, and then click Save changes. Administrator permission required If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
This is probably a permissions issue; add a folder to the server, share it with everyone (sharing and security if safe to do so) and then try. Since you've tried this and it worked, it means the problem is with the permissions you've got set up/configured.