Windows 7 show it’s network status (next to the time indication at your taskbar) as ‘No Internet Access’.
Windows 7 includes a feature called Network Connectivity Status Indicator (NCSI), which is part of a broader feature called Network Awareness which collects network connectivity information. If you are using a proxy server in your environment, the service used for network awareness is unable to authenticate itself as the currently logged on user. This is resulting in HTTP requests arriving at the proxy server as ‘anonymous’. In my case Microsoft Forefront TMG blocks these requests, which is logical as it is not a valid user.
Use the following command to set your proxy server for system services that need Internet access:
netsh winhttp set proxy IP address:port
netsh winhttp set proxy 192.168.2.1:80
If you want to push this setting for your clients, check the registry key mentioned below and export it by right clicking on the subkey Connections. Import the setting by a logon script or through group policy preferences (Windows Server 2008) under the User Configuration.
More information about NCSI: http://technet.microsoft.com/en-us/library/cc732049(WS.10).aspx