Windows has a cool feature that allows you to tell if your computer has Internet connectivity when you are connected to a network (see image below). This feature is called Network Connectivity Status Indicator (NCSI) it uses a combination of DNS and/or HTTP look ups to tell if you are connected to the Internet. The way does this is either via a HTTP request for http://www.msftncsi.com/ncsi.txt or a DNS look up for dns.msftncsi.com that resoles to 188.8.131.52
However if you find this error message really annoying there is now a Windows 7 group policy will turn it off. This is a machine setting so edit a Group Policy Object that is applied to all the workstations you want to turn this message off. Then navigate to Computer Configuration > Policies > Administrative Templates > Network Connections and enabled the “Do not show the “local access only” network icon” policy setting.
TADA… Now you will no longer see the exclamation icon on the network icon.
For more information on how NCSI works and this Windows 7 policy see http://technet.microsoft.com/en-us/library/ee126135(WS.10).aspx
Unfortunately Windows Vista does not have the same Group Policy however there is a registry key that can be applied using Group Policy Preferences that has the same affect.
Data: 1 (REG_DWORD) = Enabled
Data: 0 = Disabled
Step 1. Edit a Group Policy Object that is applied to all the workstation you want this Browser Ballot disabled.
Step 2. Navigate to Computer Configuration > Preferences > Windows Settings > Registry and create a “New Registry Item”
Step 3. Type “SYSTEM\CurrentControlSet\Services\NlaSvc\Parameters\Internet” in the Key Path then type “EnableActiveProbing” in the Value name, then select REG_DWORD as the value type “0” in the value data and then click “OK”.
For more information on how NCSI works and this Windows Vista policy see http://technet.microsoft.com/en-us/library/cc766017(WS.10).aspx