If you're using WEP encryption and managing your wireless networking with Windows Wireless Zero Configuration, follow these steps:
1) Open the Network Connections folder ("Start > Run", type ncpa.cpl and click OK)
2) Right-click the Wireless Network Connection and select Properties.
3) Click the "Wireless Networks" tab, select your network in the Preferred Networks section and click the Properties button.
4) In the next properties sheet that opens, select the Authentication tab and untick the box "Enable IEEE 802.11x authentication for this network".