Vista Wireless Configuration and Troubleshooting
Step 20: Remove Preferred Networks
Return back to Control Panel and double-click Network and Sharing Center. If you do not see the Network and Sharing Center icon, click the Classic View link in the left sidebar.
In the window that pops up, click on Manage Wireless Networks link in the top left corner.
In the next window that pops up, right-click on a network and click Remove Network. Repeat this step until you remove all the networks from the list.