Go into the Network Connections folder ("Start > Run", type ncpa.cpl and click OK), right click the network adapter (Wireless Network Connection) and select Properties.
In the properties, there should be...
- Client for Microsoft Networks
- File and Printer Sharing for Microsoft Networks
...and both should be ticked. If either is missing, click the Install button and, in the box that opens, select Client to reinstall the missing item(s).