Windows 7: Laptop won't autoconnect to network after sleep

I searched this forum and found several similar posts but no solution as of yet. My Dell Inspiron laptop running Windows 7 with all updates and service pack is set to connect to my wireless home network automatically. But if I close the lid or turn off the computer, it won't connect to network automatically when I restart. I check the network and sharing properties and find the "connect automatically" checkbox is now unchecked! What causes the system to change this setting and how can I prevent this?