Hi
This happens to my once in a great while, and what I've had to do is bring up the Start Menu, go to All Programs - Accessories - Communications - Network Connections, then in the Network Connections window - click on the Wireless Network Connection, click on File for the drop down menu, then Properties, your going to get the next window with (or at least I get) General - Wireless Networks - Advanced, click Wireless Networks and in that window - check the box that says 'Use windows to configure my wireless network settings, then click 'OK', this has helped my many times, give it a try.
William