After installing Avast, I noticed that Windows 7 couldn't detect when there was an internet connection any longer and I was perpetually warned of "No Internet Access" and a little yellow icon on the wireless icon in the tray. After a little googling I found THIS demonstrating how Win7 checks for connectivity. After a little testing i got the problem solved. To get to the point, go to Realtime Shields > Web Shield > Expert Settings > Exclusions and add *.msftncsi.com/* to the list of 'URLs to exclude' and the problem will go away. Windows 7 uses this domain to check for Internet connections. I've heard other AV software can do the same, it might be good to add this info to any AV setup guides here.